The only way to gain skill at backing a boat is through experience.

Best thing a person unskilled at backing a trailer can do is to set out a "practice
ramp" using cones, buckets .....whatever. Pick a spot in an empty parking lot,
that way there's nothing to hit.

Don't over-steer, and learn to use your mirrors..that "open the door and watch"
method leaves you completely blind on the passenger side.

When I was 18-19 I'd hang out at the big public launch ramps in Newport Beach,
and could make $20 a pop all day long just backing peoples boats/trailers in
for them .