Two methods from past discussions
Long detailed discussions of this somewhere in the archives, we need some kind of index for subject matter. Anyway, one method involves hoisting each end by some means, like a scaffold or barn stringer, until the trailer is rolled out and the boat blocked up. The other involves jacking up the whole trailer and boat, blocking the boat up, and then letting the trailer down. Or combinations of these, depending on what facilities are available. Would help to know what kind and size of boat you are talking about.