Mod: https://www.moddb.com/mods/afraid-of-monsters-dc/downloads/afraid-of-monsters-directors-cut

Run the EXE file and install the game on a new folder.
Rename the folder to "AOM".
Move the folder to the "C:\Program Files\(x86)\Steam\steamapps\common\Half-Life" folder.
Go to the Half Life properties configuration and type in the launch option: -game AOM
After doing this, just start Half Life and the mod will run
Update:
To resolve the compatibility issue with the 25th anniversary update, go to Half-Life settings>Betas> and select the version "steam_legacy - pre 25th Anniversary Build"