drop the folder "Z_classic_monsters_V2" into your DOOM 3 game folder, next to the "base" folder, like this:

../your doom 3 folder/DOOM3.exe
../your doom 3 folder/base/
../your doom 3 folder/Z_Classic_Monsters_v2/

then, you can run the file Play.BAT to launch the mod or enabling the mod in game loading the mod "Z_Classic_Monsters_v2" from
the "mods" option in the game Main Menu.

Each .PK4 file inside the folder "Z_Classic_Monsters_v2" haves 1 custom monster. Please Read each _Readme.txt text file.
You can also drag and drop one or all custom monsters .pk4 files to your custom mod folder and make custom maps with them. Please read each file
_readme.txt because there is the information about each monstser pack, credits, specifications and more info. And remember add the authors in the
credits of you use the monster pack. You can preview the monsters in the folder "previews", and see how the monsters look. have fun

NOTES:

The folder "low_res_textures_and_models" includes 2 .pk4 files. the file "Z_zzClassic_lowres_textures.pk4" includes low quality textues for
SOME of the monsters included in this mod, this can help if you are playing in a slow device. Drop that file next to the other mod "pk4" files.
the file "Z_zzClassic_lowres_models.pk4" forces a low poly model for some of the monsters included in this pack. If you want to play with
more FPS you can drag and drop that .pk4 file next to the other "pk4" files from this mod.

The folder "disable_screen_shake_explosions_and_monsters" includes a ".pk4" file that if you drop it in the main "classic monster v2" mod folder
will disable almost ALL the screen shakes that are caused from explosions, monster screams and footsteps. Added instructions also

The folder "af" includes some "ragdolls" that can be used for some specific monsters from DOOM 3 and custom mods. Can be useful for mod developers

The folder "preview" includes some JPG image previews from the monsters included in this addon. Also the images are named with the same spawnClass from the monster referenced.