If you can't find it on the wiki (floor textures):
You have to create a marker, called F_START. Use the new entry thing. Then, go and find any image you want to use as your floor. It must be a bitmap file, and it has to fit in the requirements. (forgot) Then, put it after F_START. Then, click on your image and click on FLATS. Find SAVE AS DOOM FLAT and click on it. Then, if you got all of your textures in like that, add a F_END marker.
It was something like that. I did it in a weird order when I tried it myself.