Remove the racks, spray them with a degreaser and leave them to one side. Warm the oven to 50 degrees. Spray the oven with any oven cleaner, leave it for about 5 minutes, then with a small scraper, similar to a stanley knife blade, scrape all the black residue of the oven and wipe it with paper towel. Afterwards just wipe the excess cleaner off with a damp cloth. It takes me a maximum of fifteen minutes and you don't need to drown the place with the dirty oven cleaner water at the end. I used to hate cleaning the oven but this way it doesn't feel like such a great chore.