You just need three things that you most likely already have in your kitchen.
1. Sprinkle baking soda onto the baking sheet
2. Apply a layer of hydrogen peroxide
3. Add more baking soda on top
4. Let your mixture sit for up to two hours
5. Wipe it away using a coarse sponge or rag
I recommend testing it out on a small spot on the back of the sheet.