First, install fallout 2 on your computer, then create a new folder in the Fallout 2 directory with any name.
Add mod files into the folder.
Make sure that you have the Fallout2.exe AND fallout2.cfg file in the mod folder.
That's it. If you now use the Fallout2.exe in the mod folder, it takes the critter.dat and master.dat from the previous folder (Fallout 2 installation) and uses all new files from the mod folder.
Now change the entries for critter.dat, master.dat, patches, music, etc. in the fallout2.cfg of the mod folder like in the following example:
That's it. If you now use the Fallout2.exe in the mod folder, it takes the critter.dat and master.dat from the previous folder (Fallout 2 installation) and uses all new files from the mod folder.