Backing straight or making a right angle turn to back into a slip or Med moor are the only maneuvers that a single engine boat doesn't really want to do. Practice a few times in calm conditions, and you will get the hang of it, then try in more challenging conditions and you will likely find some of your technique works just as well, some not at all, so you will learn what does work.
Speaking generally, a rudder is designed to operate mostly (power boat, not sailboat) on prop wash. That design feature should be kept firmly in mind, so you are not tempted to try to use the rudder when there is no prop wash acting on it. ie: don't try steering with the rudder while your prop is turning in reverse. In reverse, a much stronger effect on your steering is the walking of the prop. A right handed prop will walk to starboard in forward and to port in reverse. In forward the walk is cancelled out by the rudder reacting to the prop wash. In reverse the walking is going to move the stern of your boat in the prop wash direction.
With that understanding, you can put the stern of your boat exactly where you want to, provided you don't ever try to fight the prop walk. So, if you need to moor on a slip that is on your port side as you approach, and you have a right handed prop, with a walk to port in reverse, you can back in by putting your helm hard to starboard and using fwd and reverse to move your stern around and get some way on in the reverse direction. With the rudder remaining hard over, if your stern needs to move further to port, just give a burst of forward and it will move over, returning to reverse for the movement back into the slip. If you are too far to port, you will need to put your helm to port and give that burst of forward, then continue in reverse.
If your slip is to starboard, and you have the same port moving stern, go in bow first and your boat will naturally want to turn into the slip, keep your helm over to starboard and bursts of reverse will kick the stern away from the dock till the boat is facing straight in, then you can just steer in.
Don't try to back into that starboard side slip, and only go bow first into the port facing slip if you can complete your turn before entering the slip.
The same technique will work on twins, you just have twice the props and rudders, so can do both fwd and reverse at once.
On either single or twins, it is most important to put the bow where you want it first, while you have complete control of the stern, where the action is. Don't ever give up control of the stern by handing some dockie a line, if the bow isn't already where you want it. No lines should be handed to helping hands until the boat is fully where you want it to be, unless you don't want any credit for docking the boat and want the dockies to take all the credit for bringing you in.
Above all, practice in a variety of conditions.
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