Tough to give you real good advice without seeing exactly what your slip looks like and being aware of local conditions. But you're wise to think ahead about the best way to approach it before you get on the water, especially singlehanded with a single engine boat with a straight transom that's almost impossible to track in a straight line while in reverse. I suggest that you go to your marina and just look at local conditions and all the various ways other people put themselves in precarious positions as they arrive and depart and then spend a lot of time thinking about what you see and how to avoid their mistakes. Look for a sale on fenders and preposition them all along the sides of your slip, cheap insurance for when the best laid plans inevitably go awry.
From what I know about your situation, I wouldn't attempt to back into a slip under power at all, but I would leave your engine running, just in case you have a sudden change of plans and need to reset and ponder what just went wrong. Preparation (physical AND mental) is everything. As you approach the slip, make sure you have fenders hung at the proper height and don't scrimp on size. I would rig docklines fore and aft, making sure your bow line is long enough to reach about 10' beyond your stern routed outside your bow pulpit. Also,if you don't already have one, consider adding a midship cleat so you can have a third dockline rigged there. As you approach your slip, be especially alert for current conditions and any changes from "normal" such as current, crosswinds, other nearby boats protruding further than normal from their slips, other boats preparing to arrive or depart from nearby, waves, etc. If you have passengers, mention to them that you'll have to take a break from whatever you're discussing so you can really focus for just a couple of minutes. Then, just pull up (or back up) to the end of your slip and with both (or all 3) docklines in your hand, step off onto your slip and walk your boat into place. With all 3 lines in your hand, you will have control even in a crosswind. Your other option is to remain in your boat and if there are sufficient handholds along the edge of your slip, walk it in that way, but in strong winds or current this may be impossible. If an especially strong gust comes up so it's difficult to control (unlikely with a 20' boat), quickly TAKE A WRAP around a cleat or post until you can get some help, don't stand there slowly getting pulled into the water. If your slip is one of those that only extends about 10' with a post further out, tightly string a permanent line from the post to the end of your slip so you can grab it while still in our boat and walk it in that way. If someone is there to help you, don't assume they know what they're doing, no matter how well intended they may be. Most helpers will tend to yank hard on whatever line you hand them with no thought of what that will do to the other end of your boat or how difficult it is to get a heavy boat stopped before it smashes into something, so if someone offers to help, stick to YOUR PLAN and be polite and calm, but DIRECTIVE! Have an additional fender available in your boat with lines rigged from both ends that you can quickly grab and place between your boat and whatever it's about to hit, but on a smaller boat, a life vest can also be used, much better than jamming your fingers or scratching your neighbors boat when something unexpected happens! I like to have lines rigged from your slip that are premeasured to keep your boat from hitting anything and quickly slip the loops in the end of these lines over your cleats once your boat is in position. The docklines you used during the docking process can be used as backups or coiled up and stowed. You can look in Chapman's or many other sources (BoatUS) to reference how to set up bow, stern, and 2 springs, and on a slip with access to both sides, if you're anticipating bad weather, secure both sides. When you're initially tying up your boat, keep in mind that if yours is not a floating dock, even a small amount of tide will affect the tightness of docklines so you should anticipate that and plan accordingly. I realize I mentioned a lot of things here and many of them may not apply to your situation, but hopefully they'll get you thinking about which of them might apply to your particular slip and boat.
You're already off to a good start because you're thinking about it before you even get on the water. Good luck!