For mods, you need to grab some mods, of course. Most of the ones on this page
are known to work well with nxDoom.
Anyway, download the mods that you want. Each mod should have one or more WAD file, and maybe a DEH file.
You need to create a file called "modlist.txt" (without the quote marks), and place it in the doom directory on the CD (the same place as the main WAD files from Doom).
Inside the modlist, you all one line for each mod. Each line contains the name of the mod, the filename of the main WAD file, a list of command-line paramaters to give to Doom, which tell it to add additional WAD files (or DEH files), and a description (which is never used). Each line should look like this:
Name of mod,doom2.wad,-file extrafile.wad extrafile2.wad extrafile3.deh,Description
Note that there are no spaces around the commas. If you do put spaces around them, they will break.
If the mod is for Doom, or Ultimate Doom, instead of Doom 2, you should put doom.wad or doomu.wad in the modlist line instead of doom2.wad
The bit after the -file should have a list of all the WAD and DEH files that came with the mod, separated by spaces.
For example, you have a mod called "Evil Modification 2" for Doom 2, which consists of the file evilmod2.wad and the DeHacked patch evilmod2.deh, your line would look something like this:
Evil Modification 2,doom2.wad,-file evilmod2.wad evilmod2.deh,Evil Modification 2 for Doom 2
Example 2 - You have a mod called "Stupidity" for Doom, which has two WAD files (stupid_levels.wad and stupid_gfx.wad), and one DeHacked patch (stupid.deh):
Stupidity,doom.wad,-file stupid_levels.wad stupid_gfx.wad stupid.deh,Stupidity for Doom 1
Example 3 - You have a mod called "ExtraMaps", for Doom, you want to use the Ultimate Doom WAD file (four episode version), and it contains only a WAD file (maps0001.wad), your line would look like:
ExtraMaps,doomu.wad,-file maps0001.wad,ExtraMaps for Ultimate Doom