You're wrong Silent...
1:Make a lump called "F_START"(Not sure)
2:Add your floor textures.
3:Make a lump called "F_END"(Not sure)
Note:I was not sure if it was "F"
1:Load something called"TEXTURE1.LMP"And"PNAMES.LMP"That are found on an article on the Wiki(But i'll PM both of them to you ok?)
2:Make a lump called "P_START"
3:Add your wall textures.
4:Right-Click and click in"Add to PNAMES"And then Right-Click again and click in"Add to TEXTURE"In that same order.
5:Make a lump called"P_END"
And you're done!
Just to let you know, the lumps that are supposed to surround the flats are supposed to have 2 'F's in the name, not 1. And you should try using "Save as Doom Flat" for flats that are 128x128 or bigger, so they won't appear strange in-game.
The height and width of a flat must be a power of 2 and be equal.
The width of a texture must be a power of 2, but the height can be any value up to 255. If you want to make wall textures with a size of 256x256, 512x512 or 1024x1024, etc, use multiple patchs for them.