Steel Wool and elbow grease. Warm water, a little soap on the scouring pad and scrub scrub scrub til you get rid of the surface rust.
Once you get rid of all the surface rust rinse it out completely and dry with paper towels. Light spray with avocado oil or something with a high smoke point and into a 500 degree pre-heated oven to season. Make sure there’s just a light coating of oil.
Fits on a 22 inch Weber kettle with space to access the flip up grates to add coals on either side of the pan when it’s centered.