How do you put custom graphics to a level?

[RoundPiplup]

I see levels that have unique/different kinds of graphics. I downloaded some of those, but I don't know how to do it. Any help?

[FanofSMBX]

Step 1: make a folder that's the same name of your level file (grass.lvl -> grass), and put it with the level file in the folder
Step 2: put them in there
Step 3: save your level

[RoundPiplup]

Thank you FanofSMBX! Now I can put altered blocks/backgrounds/NPCs.