I learned the basics of backing in using the same youtube video posted above, and just like the others have said practice is everything. I'm for sure a rookie at backing, but it's the only way we go now as it makes getting on/off the boat easier. It's sort of like riding a bike in that once you figure it out.....
Being single screw and with no thruster ( it's in the boat just not connected) I prefer to back to port. Everything is done at idle with my ruder hard over until the boat gets between the piers and we're for sure headed backwards and sort of lined up, then I'll straighten the rudder. The rudder is never touched until a quarter of the boat is between the piers. If the wind is playing a factor, I'll sometimes have to give a kick ahead to get things moving in the correct direction, but 99% of the time, I lay off the throttle.
It took me a while to focus on the fact that the stern of the boat is what's turning which controls the bow. At slow speeds backing, once you straighten the rudder, it has not much effect since the boat's moving so slow, but it does respond. Once the rudder gets straightened, while reversing, point the rudder in the direction you want the stern to go. For us, when we get far enough into the slip we get a bow line on a pier, ( preferably the same side as the finger pier ), and at that point holding her in reverse will bring the stern up against the finger pier, and then we start getting lines where they need to be. Our rule is no jumping to finger piers...have to be close enough to step off.
Every so often all the stars line up and I can get her in on the first try, but I'll for sure abort the attempt and come back around if things are not going smooth enough. I"ll do a standing turn in the fairway to ensure we're going in to port, but sometimes we have to back to starboard, for which because of the prop walk, she's not as easy to massage.
That youtube video was spot on for me in regard as to the fundamentals of this skill, and gave me a good base line for having a mental check list in my mind as I bring the boat down the fairway.... stop the boat...begin turn...put stern quarter on line with pier....forward...reverse....forward...reverse...you get the idea. I will say that I'm a happy captain when we're coming into the harbor with the flags pointing straight down, and my neighbor is standing on the pier with waving to me waiting to catch a line.
Conall