Choose a compatible starting point
Identify your Minecraft version and confirm the pack actually uses Forge. On the official Placebo file list, select a normal mod jar tagged for that Minecraft release and loader. Check the required Placebo range in your dependent mod's file details before downloading.
Install into the active instance
Close the game, open the launcher profile's game directory and locate or create its mods folder. Place the intact jar inside it alongside the dependent mods. Do not extract the jar, use a sources archive or assume every profile shares the default game directory.
Launch and check
Start the Forge profile and read any dependency message carefully. If the game asks for a different range, replace the incompatible jar rather than keeping both. Preserve your previous working pack so you can restore it if a wider update introduces conflicts.