Well, if you are adding a custom texture, be sure that it is either a floor, or a wall texture, as there is a difference.
For floors, it's a lot easier. Just create a spacer (edit > new > F_START and F_END) and insert the floors in the middle. All you have to do is save them as a doom flat (image > save as doom flat) and it will work. If you dont save it as a doom flat, it will look corrupted in the level you use it on.
Be cautious that as I know now, floors are 64 x 64, 128 x 128, and I think they go on like that, because my custom floors (which weren't perfect squares) got resized, and messed up.
For walls, it's a bit more difficult.
You need to make the patch spacers (P_START, P_END) and the images. Luckily, they don't need to be doom flats (and shouldn't be) and don't have to be perfect squares. Once you have them, you need to get the PNAMES patch and TEXTURE1 texture pack in the wad with the custom textures. At this point, I hope you aren't adding the textures directly to the level wad you have, and instead are using a seperate wad file for the textures.
When you insert the PNAMES and TEXTURE1 patches, (make sure the wall textures are inserted between the patch spacers, and the PNAMES and TEXTURE1 patches after P_END) right click the wall texture, and it will say "Add to PNAMES" and "Add to TEXTURE". You need to click both for that
one wall texture, and voila, it's now availible in you're wad. You will not be able to see them in Doom Builder or Wad Author unless you are using that wad as the gfx source, and I'm pretty sure you were using srb2.wad (or as I do, ssrb2.wad, because srb2.wad is 2nd in line when srb2.srb isn't present, and if srb2.wad gets corrupted somehow, so does SRB2), what you will have to do is just manually type the 1 - 8 letter texture name in the wall, and just save it without seeing the effect, but it will be there when you add the texture wad to SRB2 along with the map.
Hope that helps, I don't remember where I found PNAMES and TEXTURE1, so I uploaded them in a seperate wadfile for you to use.
Texture1 and Pnames patches download
Hope this helps! =)