Hello AKMary,
I'll have a bash at this one. Singlehanded stern tying can be done, with no issues but it takes planning and preparation as GHOST has said previously. I have done it myself perhaps about 30 times. It's a great skill to have in your arsenal of boating manoeuvres.
First, relax. Grab a beer (or a cappuccino) the night before and study the tides, depths, and anticipated scope of anchor and stern-tie required. Prepare this stuff on deck, make sure your anchor and stern-tie line is free to run - no snags, mark it out, and physically & mentally go through the mechanics of the manoeuvre. I'm usually on beer two by this time. Before I leave the deck for the night though, I have it all ready to go.
Next morning, up early and at least one cup-of-Joe in you, motor on over to your stern tie site, and leisurely drop your hook. You will be bow into dominant force here (Wind-or-Tide, as usual), and do not worry about the stern tie yet, plenty of time for that later. Set your hook good-and-tight. Use whatever scope you feel is necessary for that location. KNOW that it's set. Now you can shorten your anchor scope as required to bring your bow opposite your stern-tie position. Ensure both sticks are in neutral, and leave the engines ticking over.
Launch your dinghy and grab your pre-prepared stern-tie rope. The way I do this is, if my stern tie rope is 600', I will secure the Stern-tie rope to my boat-cleat at the 500' mark, leaving 100' in the boat to play with later. Now fake the rest of the stern-tie line into your dinghy, and row over to your rock / pad-eye / tree / ring, whatever,,, and secure it there. Do not be concerned about how much slack you have at this point, just tie that line off and you now have your boat secure at both ends.
Row back to the boat and start playing with the lines. You will jockey your boat into final position by paying out anchor and coming up on the stern-tie line. I always have a winch on my all my boats, whether power or sail, for this purpose. No matter though, if your boat cannot be handled by hand, flash up your engine and provide a short burst of hard-over port/stbd to bring your ass around where you want it, then quickly into neutral and take up slack. I do not see you jockeying like this for more than 20 mins.
When you have settled into your desired stern-tie position, (NO, not the beer just yet,,,) Go sort out your proper stern-tie line. The one you have on now is just temporary to get your butt into position. Your proper line will run from the boat, around your shore-tie anchor, and then back to the boat. Use both your port side stern cleat AND your Stbd side stern cleat if possible for a proper set.
Tighten up that stern line good-and-tight. You will soon get the hang of how much line to use and how taught to make it - every location is different, and it's also dictated by the conditions. Sometimes I have all 600' out and sometimes I'm shortened right up with only 30' out, stern line like a guitar string. Try to use Yellow Polypropylene. It high-viz and it floats. If in doubt, just take a quick glance around the anchorage. There will usually be one or two boats who have their act together, and have things set 'just so' Emulate them,,, that's what I do ; )
Remember, if you are in a tidal area,,, the tide she'll raise and she'll drop, so be prepared for that. But you already knew that from last night's prep, right?
Yes this means two row boat trips, but since everything was prepared the night before, this entire Stern-tie production takes about 35 minutes. That's not bad for a single-hander. You have been in complete control at all times, you have been able to bail at all times, and you are ready to bail now if required. Go get that beer, you have earned it.
Lastly, forget about counting on anyone's assistance. In my mind, "Single-hander" means self sufficient. You just got to be prepared to do this all by yourself. IF someone asks to lend a hand, and you need it, sure why not take advantage of that gift-horse, but do not go into this thing with a mind that someone will be there to assist, no worries,,, (guess that's another topic)
Good luck, you can do it, & let us know how you get on.