For each grate:
1 gallon zipper plastic bag
1/4 cup ammonia
Put the grate into the bag, and add ammonia. Seal the bag and wait 12 hours. Remove grate from the bag, and clean off grates with a sponge.
Another way I tried to clean grates was with a soak in washing soda and hot water. This worked somewhat, and helped reduce the grease, but the ammonia method above was cheaper and better.
Ammonia works on even the blackened, hardened fats. It didn’t clean mine entirely, but still did a visibly good job.