First things first
Remove the racks. Using a spatula, scrape up as much of the debris as you can, and throw it away. Most of the big stuff should come up pretty easy. Tip: move the trash can closer to the oven....
Using about a cup of baking soda, more or less depending on the size of your oven, sprinkle it all over the floor of your oven.
This is where a squirt bottle comes in handy. Spray vinegar all over the baking soda. Use your fingers or a rubber spatula to spread the baking soda around so that it comes into contact with the vinegar and every dirty part of your oven floor. Let sit for 10-20 minutes.
(alternative: Mix vinegar and baking soda in a bowl to form a paste, and then spread that over your oven floor. Make sure to pour the vinegar slowly because it fizzes up pretty high.)