3/8" Threaded Leveling Feet Adjustable Table/Furniture Feet Levelers Screw on Heavy Duty Desk/Chair/Cabinet/Dresser/Bench Leg Leveler Screw in with Threaded Inserts(3/8-16,Large Base,Black,4 Pack)

Find the two legs that are the longest. They will be the ones it rocks on. On one of those start filling down the bottom little by little with a rasp or sander until it sits flat. Alternatively you could put adjustable furniture feet on the legs, but those may not work given the size of the chair. Something like this https://a.co/d/01nZB73j