You can use a trash bag, but you have to be careful to avoid poking holes in it. Or you can use a plastic bin with a tight lid. Put the grates in and pour ammonia over them, then close tightly. Let sit overnight. Rinse the grates and put them back on the stove.
If you cook a pie or anything that spills, watch for the spill. Open the door and bury it with salt. Shut the door and keep cooking. When the oven cools you can sweep it out.