Doom 64 Weapon Pack v1.0 file

Step 1: Download the file.

Step 2: Extract the file with a program like 7-Zip or WinRAR.

Step 3: Buy The Ultimate Doom from steam.

Step 4: Go to your DOOM directory, and copy your wad file (DOOM.WAD).

Step 5: Download GZDoom.

Step 6: Put your Ultimate Doom WAD in the GZDoom folder.

Step 7: Drag WEAPPACK.WAD in the GZDoom folder.

Step 8: Drag WEAPPACK.WAD onto the GZDoom executable.

After that, you're good to go.