I know this subject has been touched on before, but I thought I'd do a refresher. I needed to adjust the trailer rollers & not having a convenient launch to hand, this is how I do it.
1. Fix to boat to a solid, hopefully immovable object
2, remove all straps & winch, drive slowly forward pulling the trailer out from under the boat.
3, as soon as the transom clears the trailer, place well padded blocks under the transom.
4, drive further forward, blocking as you go.
5, clear the trailer from the boat.
6, to replace the boat onto the trailer, back the trailer up to the boat & attach the winch, put vehicle in neutral & handbrake off. Attach winch & winch the trailer back under the boat pulling the vehicle with it. Remove the blocks as you go.