- Download the AddOn by clicking the "Download ZIP" button
- Extract the downloaded ZIP
- Rename the innermost folder (containing the AddOn files) to
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- Copy that folder into your WoW\Interface\AddOns folder
Open core.lua in a text editor (one with Lua syntax highlighting is preferred, but not required) and add entries to the SPELL_IDS table for any spell you want to play a crit sound for.
You can also change the SOUND_FILE setting to play a different sound file.