Re: How to backing down the ramp
As others have said, practice in a parking lot. Take a couple of traffic cones or big cardboard boxes to make a lane to back through, so you get some practice backing in confined spaces.
Remember that small actions of your steering wheel are usually all that's needed. Most everyone oversteers when backing a trailer early on in their experience.
You don't say what the tow vehicle is, but if vision is restricted behind it, practice using your mirrors for backing. If you have something like a minivan or suv with a rear hatch, open it, so you can see your boat on the trailer. I can't even see my trailer when it's empty without opening the hatch on my suv.
You may encounter ramps with all sorts of difficulties. Some don't have a long run in front of the ramp, some make you start at an angle. Try all those things when you're practicing.
Whether you're practicing or at the ramp, if your trailer gets too far out of line, never be afraid to shift to a forward gear and pull forward to straighten it out. Everyone has to do it from time to time, so just bite the bullet and pull forward if you need to.