The fabric of your jersey gets slightly stretched during construction; heat reduces the fabric back to its normal size. Therefore, the way to shrink your football jersey is to run it through the washing machine using hot water. A normal wash cycle should be satisfactory. Use laundry detergent according to directions on the bottle, the soap only helps to cleanse the shirt, it does not help or prevent the shirt to shrink. Tumble dry the jersey on high heat following the wash; make sure the jersey is completely dry before removing it from the dryer. The jersey may be washed with or without other clothing.
Football jerseys made of pre-shrunk cotton likely will remain the same size no matter how they are washed. Fabric softener can damage some fabrics or fabric blends. Refer to the washing directions on the label on your jersey before using fabric softener.