As others have said, buying one is probably a better choice.

However, we are just finishing building a trailer for our boat. The reason
is we could not find one to purchase that would have out boat set as low to
the ground as we wanted it to. If you still want to build your own trailer
I'd be very happy to share what I've learned from building ours. I designed
ours myself. It came our very well. So far there is only one thing I'd
change and even that is not a major problem.

Gary