1. Unzip ImprovedDropRates.zip and extract ImprovedDropRates.pak
2. Go to your Bloodstained game folder
3. Go to BloodstainedRotN\Content\Paks
4. Add a folder named ~mods (if it doesn't exist yet)
4. Add ImprovedDropRates.pak to this folder

To remove the mod, just delete ImprovedDropRates.pak