In SMBX2, how do I create custom tilesets?

[MarchTenth]

In SMBX2, how do I create custom tilesets without changing the normal ones?

[Hoeloe]

You can use the blocks between ID 751 and 1000, but I would suggest you don't bother. If you're using all 1000 block types in your level, it's likely to already be a mess, so I suggest just replacing one set you aren't using.