Put all files into the doom2 directory.
Then at the dos prompt goto the doom2 directory and type (leave out quotes)
"necro1" the game should start up. 

IF you choose to add all sprites to the wad, instead of typing necro1
type necro2. after that each time you want to play type
"doom2 -file necropol.wad" at dos prompt.
for those who want, NOGFXNEC.WAD is the same wad without the new sprites so
just type the above doom command to start it.