There are two ways to do this depending on whether you are reversing into position or going forward to a halt. In each case the engine direction is in opposition to the placement of the line – for a reverse approach you use a forward spring loop, for a forward approach you use an aft spring loop.
First prepare the line you will be using. Attach a line to your aft cleat. Bring the other end of your line to the midships/waist cleat with the line almost touching the water. Mark it or cut it so that you can quickly tie it off to the right length. This is an “aft spring loop” and will be your docking line for forward approaches .
Prepare an aft spring loop on the appropriate side. Approach slowly in forward gear, turn 30 degrees to the dock until you judge you are getting close, then straighten the wheel. You should be in neutral by now. Make adjustments (reverse if you’re going too fast, forward if you’ve lost way) and slow down until you are stopped or nearly stopped a few feet away from the dock. Ask the crew to toss the aft spring loop onto a cleat. Then pop into forward with very low throttle and turn the wheel away from the dock. The boat will be sucked into the dock and held there so that crew can safely and slowly disembark. If you need to do a lengthy fuel up or need to take your boat out of gear for another reason then the crew can secure the boat normally with mooring lines. Otherwise you can just stay in gear while you pump out.
As for above, first prepare your spring loop. You may be able to use the same line for aft or forward spring loops, or they may be of somewhat different lengths. Attach a line to your bow cleat. Bring the other end of your line to the midships/waist cleat with the line almost touching the water. Mark it or cut it so that you can quickly tie it off to the right length. This is an “forward spring loop” and will be your docking line for reverse approaches .
This approach is very similar to above but you will be backing in and using a forward spring loop. The preference is also to land on the port side if you back to left (or starboard side if you back to the right). However with this method you can approach with either side to the dock in good conditions (little wind or current). Once the spring is deployed, pop into a gentle reverse, and you will be sucked into the dock and against the spring line.
You can also use this method if you are going in forward but the only cleat is ahead. Then you can use the forward spring but change into reverse and go backwards to secure yourself against the dock.
For more information about Docking, please see the full article on Tips for Docking a Sailboat Safely.