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Part 1: Fundamentals And Basics
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So okay, firstably experiment with stuff, its okay if ou dont have ideas and go straight to the point. Think about wha goals and stuff you want to make in your episode. Do you want to make a SMB3 themed episode? Just find whats fitting to you or the ideas you have.
Secondly, make sure you download SMBX2. Also optional if you want to use the old ver (SMBX Legacy) Its recommended to download SMBX2 since its really the most stable version and also has a configurable and user friendly UI. If you want to make SMBX episodes for SMBX Legacy then you can use PGE and download SMBX 1.3 config and start making your dreams!
Okay, thirdly, open up SMBX on your files. Now click the Editor button (level editor for SMBX Legacy) and wait until there is a popup with a white and grey/black and blue window. Once you do, you should see some two icons at the top left corner of the window. it should look like this:

you know the drill, paper icon is New... And files is Load...
if you want to make a episode/level, click on the paper.

and then click on the world icon (the one with the earth icon) and for levels, click on the fortress icon.
Since we finnaly got the basics,we can start fiddling and do whatever the fuck you want!
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Part 2: The Worldbuilding and Stuff
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Since we finally done part 1, lets do worldbuilding!
Okay, once you did click on the world icon, you can work on some various things to start with:
- Episode Settings
- Toolbox and World Map Creation
EPISODE SETTINGS

In order to jumble your own proper episode, you can click the cogs shown in that screenshot.

if you clicked that icon, a popup should appear. the very top is a bar where you can name your own episode/world.
Next is a couple of options like :
-hub styled world (checking this option makes the world completley inacsessible and is obligatory to have a intro level/hub if using this feature otherwise jank happens)
-restart last level after death (self explanatory)
Next is a bar with the name intro level. Just put the level the player is originally going to start with when starting the episode. If this is blank, it will normally plop you in the world map. Obligtory if using hub styled episode feature.
Total stars is the amount of collectable stars (not to be confused with normal stars that give you invincibillity for a few seconds) you have in your episode. You can let SMBX recount it automatically. SMBX2 exclusive btw

Additional Episodes Credits is self explanatory. Maximum 5 lines of words because the engine freaks out or smth look im not Redigit-
-your credits here
-a compliment for finishing a episode (optional)
-whatever the fuck you want to put here as long its not more than 5 lines of characters
And lastly disabling characters is also self explanatory, basically you can disable a character to not be usable for the rest of the epsiode unless you put char blocks or smth
also if you let link open almost every X2 exclusive characters are available too (you can fix it by just plopping some lua code, aka magic in the form of computer enginerring)
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Part 3: Toolbox and Level Settings
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Sup! i havent updated this in a short while, so here is a continuation about the guide. After with the settings stuff you might see a tab in the left side of your window. That tab is called "Tileset Item Box" and you can simply use whatever it offers. It is different based on what mode you are on (world map mode, only the map tab is available, level making mode, uses almost all tabs used but "Map" is not available.)
Firstably, you should experiment with it, as it probably can give you some ideas from it. Have fun with it. Or dont, do whatever you want. That is the important thing you should put up your mind. Here is what should it look like:

basic stuff. you got levels, paths, scenery and tiles up in the tabs and there in general.
You should start lanscaping clicking the "Tiles" tab as it should establish the territory and the levels that reflect with the rest of the land mass.
you should see SMW tiles and if you scroll down enough, you can find SMB3 tiles too. Be careful, as there is limits in term of making world maps, like: you cant make SMB3 Desert rivers exit to the water, which is lame (especially if its close to water mass) so you either need some custom graphics (which i will explain further and later in the lifespan of this thread) or make a scenery hide it.
Once you finished landscaping and play with themes and items, you should probably put some scenery in it (from the scenery tab). Dont worry if the world map looks garbage, we all have been begginers at one point or another. Heres my example:

its a simple SMW ified island with a coast, SMW Scenery, a little mountain, and a sunken board of abandoned ships in southeast of the map. This is one of many ways to make a map, and this as a example, might give you the vision about making world maps. You should take sceneries and tiles and take it like seasoning a pizza. When you make the pizza, what stuff you want to add it? its the same thing about here.
Now about levels, they are a reflection of a level in a world map. You can make the item make you enter one and play a level.
there are settings about it too, so be aware of it as it is crucial for making actual episodes.

Mostly, the position is self explanatory, tells you where the level´s coordinates.
Next is some couple of checks. some might be crucial to making the episode.
- Background Path (applies a little patch of path at the bottom of the level graphics. Useful for levels that dont use the path id.)
- Bigger Background Path (same thing but patch streches to the left and right away from the center. Usually used for castles, fortresses, blah blah blah.)
- Always Visible (Makes the level visible even if the level is not connected by paths, which if unchecked, make the level not visible until path connects through. Useful in case you want to use useles level as sceneries. Example: a town represented with multiple mushroom houses, pipes used for a pipe asthetic for a land mass, etc...)
- Game Start Point (starts the game on a certain level. Fits with the SMB3 start level, white castles, yoshi houses, etc... IS NEEDED FOR THE EPISODE TO BE PLAYABLE unless it is a hub episode.)
and then is the level file. you either type the level like "(level name here).lvl" or browse the files inside the episode´s folder.
Next is level title. you type the title for it to appear when acsessing the level. Make sure the level title is not misleading or clickbaity, as it may confuse players when entering the level.
Enter to door is basically put a number that matches the exit warp door. If this confuses you, i will talk about it later in this thread, as this is advanced for begginers.
An then is the open path by exit type: the level could open a diferent path depending on what type of goals you introduced in your level. For example:

Lets say i have a level with a secret exit and a normal exit. I have diferent goals and exits in the level. My secret exit is a key hole in that level, which i select 4 - Secret Exit. If I want to continue going in the normal path, renember what normal exit you had in the level. is it a SMB3 Exit? Or is it a Power Star? Select what coicidentally fits with the exit in your level. I have a SMW goalpole, so i use 9 - Tape Exit.
and finally go to coordinates. Useful only when making warp stations to other parts of the world. You want to teleport to the other part of the world map, then make a coordinate to the destination you want the player to be in. If you want the first part to be acsessible and the second part as well, then i recommend making pipes that coordinate eachother.
Level settings just took half of this post, so the final thing im covering today is Paths and Music Boxes. Paths are little paths that connect to one destination to another. And music boxes are unvisible tiles you can use to change the music to fit a setting. You shouldnt put music boxes everywhere not unless its the start level, a transition area to a world, and the last level and pipes or transports. There is a lot to choose from, but it is enough to make a epiode. If you want to have custom music in it, then i suggest try using Jukebox.lua a shot.
Sigh... it has been 1 month and a half, but that dosent matter, i am back on my heels and i will update this again because why not
we should be finishing the world map part with Custom graphics and other settings, so be prepared!
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Part 4: Custom World Map Graphics and custom stuff in general
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okay, while this is optional, it should be recommended and noted in general, so basically on all versions of SMBX, they all support custom graphics and such (custom music, sounds, hardcoded...) the only thing you should do is getting sources from other games or from the community, for example, i recommend downloading from one of these:
SMBX CGFX Pack made by Red Yoshi and supermarioman
The Spriters Resource, The Sound Resource, blah blah blah...
h2643's GFX Pack
and once getting them and converting them into SMBX supported imgs which i will explain soon, i recommend downloading and crediting the community's work and sprites and graphics. had done that and drop it into your levels or episode folder. (NOTE: Putting it in the episode or world folder might cause it to be used and look as the same file rather than the vanilla and can be used in levels without copying and pasting it in the folder, and oh, dont put them in your data folder or replace the vanilla graphics at all)
once you drop them, you can use them freely in whichever place you want!
now renember when i said converting sprites to SMBX supported imgs? well heres the explanation on it. the whole thing is going to be seperated in two segments, Level supported and World Map Supported, which one cannot be compatible to the other segment not unless its converted to that named img.
level supported names imgs
block_XXXX - self explanatory, and also can be semisolid sometimes, with probably settings changing or smth (X is replaced with numbers, duh)
bgo_XXXX - background that serves as decorations or to spruce up some stuff. Not to be confused with background2
npc - graphics that are used in npcs, whether friendly or not, and whether its a living being or a platform. it will need replacing code to a npc in order to work as the graphic or generally intended
effects and particles - self explanatory, you shouldnt really modify them not unless you want to change it with a custom graphic. Particles is SMBX2 exclusive
world map supported names imgs
tiles - they function like static images or animated. Mostly serves as to signify what enviroment youre in or the world has.
scenery - the world map equivalent of bgos.
path - images that form paths to levels.
level - image to serve as a beforeview of the actual level
player - not to be confused with playables, serves as a map pointer disguised as the player youre playing as.
extra stuff to note
hardcoded - self explanatory?
mario,luigi,peach,toad and any other playable - self explanatory too, serve as players graphics
see ya