The process is very simple, you simply take off the grates and burner cover, place them in a plastic trash bag, pour in half a cup of ammonia (be mindful of the fumes), and tie it up. The fumes work on the burnt spots, loosening them. I left them in the bag for a few hours, one "recipe" I read said 30 minutes, so I scrubbed a bit after 30 minutes (with gloves on) and decided that a few hours would probably be even better (another blogger had left hers in for 4 hours). There are still a few stubborn spots but they are hardly noticeable and might actually just be worn places, so I'm really thrilled with the results. Here are my before and afters photos!
How to Clean a Gas Stove Top on Simple Organized Living & Clean Your Gas Cook Top Burner Grates on Food.com