2 ways I've done it.
We used to have a Hunter 23.5. With that boat, I dropped the bow end of the trailer as far as I could. Then supported the stern of the boat with 2 power boat stands. Then raised the bow end of the trailer up and blocked it. That raised the stern of the boat off the rails. Then I jacked the bow of the boat up, using the front of the trailer to support the jack, and blocked it. Then I could drop the bunks down for painting.
On our Seaward, I support the boat, one side at a time, using 4 , 2 x 6's angled between the rub rail and the ground. I also strap the boat against the opposite side bunk. I then drop the free bunk down for painting. Do one side, then repeat on the other. The Seaward's weight is almost all on the wing keel, anyway, and the trick is just to keep it from tipping sideways. The Hunter had to be lifted off.