Plug your sink and fill up with hot water. Add at least three squirts of dish soap and then immerse the broiler pan into the water. Allow the pan to sit in the water for at least 10 minutes.
Take the broiler pan out of the water and scrub it using a scouring pad. Add the pan back to fresh, hot water in the sink if grease still remains. This time add 2 tbsp. of ammonia and allow the pan to sit for at least 20 minutes.
Take the pan out of the water and scrub it with a scouring pad while running warm water over the pan. Make sure to not scrub too hard as you don't want to remove any enamel from the boiler pan. Repeat the process of soaking the pan until you can get all of the baked on grease removed.
Check your oven manual to see if you can run your oven's self-cleaning cycle with the broiler pan in. Make sure there is no grease remaining on the pan before you add it back into the oven. Loose grease could cause a fire during the self-cleaning cycle.
Wait for your oven to cool down after the cycle is finished. Wipe out all ash in your oven and on the broiler pan using a cool, damp cloth.