How do I properly merge different texture packs together?

Texture definition lumps are not cumulative (except with ZDoom-based ports).

The simplest solution to merge texture packs is to make your own merged pack wad with SLADE. Create a new wad, in it open the texture editor and since there's no texture yet it'll ask you what you want to do, tell it to create a new TEXTURE1 lump based on the IWAD (so you'll get the vanilla Doom II textures in it). Then open the texture editor again to actually get to the texture editor this time.

Open a texture pack you want to merge. Open its own texture editor. Select all of its textures (click on the first, then hit shift+ctrl+end and it'll select them all), hit ctrl-c to copy, go back to your merged pack's texture editor tab, go to the end (with ctrl-end) and hit ctrl-v to paste all the textures you copied from the other tab.

Repeat that process with the other texture packs you want to merge.

Save your merged pack's texture tab, close it, save the merged pack itself. There you go! SLADE will have automatically done all the other stuff needed in the background -- copying the patches and adding them to the patch table.

Now you still have to deal with the flats, but they're simpler. You won't need the texture editor for them. At the end of your merged pack, create a lump named F_START. Then create a lump named F_END after it. Then from each of the texture packs, select all their flats (they're directly lumps), copy them, and paste them in your merged pack between F_START and F_END.

If you want it to be compatible with vanilla Doom and to keep access to the vanilla Doom II flats, use "FF_START" instead of "F_START". It's a workaround to a limitation of the engine.

32 minutes ago, A.H. Sankhatayan said:

Can I rename them to keep both?

Yes.

32 minutes ago, A.H. Sankhatayan said:

Wouldn't that mess up the lumps or patches or whatever?

Not if we're talking about a texture (as defined in a TEXTURE1 lump). Potentially if we're talking about a patch.

You can use the texture editor to find what textures make use of a given patch (the texture editor has its own tabs, so look at the one for PNAMES and find the patch in question). If you rename the patch from there, I believe it'll update the textures accordingly. And then you can add the textures from the other pack that have a patch with the same name.

Thank you very much. One more thing, what should I do if 2 different packs contain 2 different textures with the same name? Can I rename them to keep both? Wouldn't that mess up the lumps or patches or whatever? 

Thanks. I did everything you said and it works almost perfectly. Only thing that doesn't are the animated textures and switches. 2 of the respacks use ANIMDEFS and the other one has Boom Switches and Boom Animated. What should I do?