Make a paste out of the soap, baking soda, and salt, adding a little water, if needed.
Apply to the interior of the oven and let sit for several hours, preferably overnight.
Dip a sponge or scrubbing brush into the warm water and wash the interior clean.
Spray the interior with some distilled white vinegar to rinse away any excess paste residue.
Notes
If you weren’t able to completely clean the inside of the oven the first go around, you might need to repeat this process one more time or spot clean any stubborn food debris.