I believe the process has been described here before, but it's a simple tho time consuming process, a couple of hours to half a day, depending on your energy level.
Use a rolling floor jack, solid blocks (wood or concrete), 2x & plywood shims.
Park the trailer tires on 2x pads. Crank down the tongue of the trailer as far as you can. Block the transom of the boat, raise the tongue enough to block it, then retract the jack & place a block under the jack foot/wheel. Jack up the tongue as high as possible. Block the bow of the boat. Lower the trailer to level.
By now, there should be enough clearance between the boat & trailer to move the trailer. It may be necessary to remove the 2x pads from under the tires to gain more clearance. Now the work begins...
The rest of the task is a process of moving the trailer forward to the bow blocking, then block again behind the trailer's cossmember/axle, then remove the forward blocking. Continue the process, moving the trailer ahead until it clears the boat. Add blocking to make four points of contact.
Reverse the process to put the trailer back under the boat.
In every step of the process, BE CAREFUL and think about what you are doing.