How do I fix tennis elbow before season starts?

Multimodal approach. Tennis elbow responds to rest, ice massage, stretching and eccentric strengthening, nsaid's (eg ibuprfen, naprosyn, (naproxen) etc), and injections. A counterforce brace can be worn as well. The easiest way to do eccentric stretching is with physical therapy. The easiest way to do it without going to pt is with a product called flexbar. I have no ownership in it. If all else fails, surgery.
Preseason exercises. I would recommend strengthening exercise for the forearm prior to the season. Also make sure that your tennis racket grip is correct and the racket weight and stringing are correct for your level of play.