The Return is a combination of things, some from TeamTNT and some from the DOOM community.
You can get the resources from TeamTNT, and then make your own levels using them. Then you can submit them to TeamTNT for inclusion on our index page, and we'll put them in our Return directory on cdrom.com, where they'll be mirrored around the world in a day or so.
What's in The Return's resources? Well, there are hundreds of textures--you saw most of them in Eternal Doom. There are sounds from Eternal Doom and the troublesome Dark Imp sprites. Lots of stuff, and it's all available for you to use in your own levels.
Go to the Index Page and see what's been submitted so far. Be sure to pay attention to what engine is needed, and any other special instructions from the author.
You'll probably need the RETRES (Return Resources) file before you can play any of those levels, so download that here.
Want to just read the RETRES text file? It's available here.
You'll need the resource file too, of course. As you design your level, all you need to do is use the new textures, and then when the players play your levels they'll include the RETRES.WAD on the command line. All you have to ship is your level (and music and stuff like that if you want to).
If you add any of your own textures, you'll need to extend the texture list in your level so it'll include DOOM2 textures and RETRES textures. We use Patcher to do that, available here. It's just a simple /X switch on the command line and you're done.
When you're ready to publish your levels, just go fill out the submission form with the information we need, and we'll write to you with instructions on what to do next to get it included.