How to Make It
Remove all the racks from the oven and hand wash them with a scrubby sponge. If they seem very covered with grease, soak them in hot, soapy water for a while—if you have a tiny apartment sink like I do, you may have to complete this step in your bathtub. If you have no bathtub and a tiny sink, just do the best you can with hand washing. If the grease won't come off the racks with dish soap, use extras of the paste you're about to smear inside the oven.
If there are any easy to remove chunks of charred food in the bottom of your oven, scrape them out and into the trash.
Mix together a paste of 1 cup baking soda and ¼-⅓ cup lukewarm water, keeping in mind that you may need more paste if you have a large or double oven. If there’s a lot of grease on the oven walls, a thicker paste may be your best bet. Use a large, clean paint brush or rubber glove-covered hands to spread the paste all over the oven, including the inside of the door. Let the paste dry a bit, then close the oven door and let it sit for 12 hours. It goes without saying but I’m going to say it anyway: Don’t use your oven while you’re cleaning it.
After 12 hours, use a spatula or scrubby sponge to scrape the hardened paste from the oven, taking the caked-on gunk with it. Fill a spray bottle with ⅓ cup white vinegar and ½ cup water, and spray any bits of paste that won’t budge with the spatula. Next, spray the whole oven with the vinegar solution, and wipe the oven again with a dish towel.
Finally, wipe the oven with a clean damp dish towel to make sure you’ve gotten rid of all the baking soda paste.