I tune with a Sears Craftsman 5/16" socket, quarter drive, a long handled allen 5/16" wrench, two rolls of coins. and a guitar tuning fork. Wear safety glasses, if you break a string it could hit your eyes. Listen to what you are doing and don't turn much at a time, that prevents you from going too far if you get the wrong string. Use a lot of light. Hold the socket straight, if you get off alignment you could damage the peg. Anybody with mechanic experience know how to hold the wrench straight on the fastener head. The pros trained by pros on here have a cat when I say that, but I've been tuning my piano 20 years and broke one A#5 string in that time, no particular reason detected. I pluck the string with my finger then follow it up with my eyes to the pin that needs adjustment. The coins are to keep the dampers off on the two notes, the one I am tuning and the reference tone. Plucking the string won't work if you are a pro, your fingernails would wear out, but I only tune one piano a quarter at the most. Don't pull up pitch a lot in one session, you have no reason to go all the way on a note that is far out, it doesn't cost you anything to come back tomorrow and go a little more.
Tuning a bit flat from a perfect fifth helps achieve equal temperment, (perfect octaves), that is tune D-A, A-E, E-B, B-F#, F#-C# etc till you work your way back to D and see how far off you are from an octave. But the top octave needs "stretch" as the overtones are out of tune with the fundamental and can confuse my ear. I can't use the middle notes as reference for the top octave. Luckily, when I bought a Hammond H100 organ, and tuned to the pure sine waves of that, i was able to quit retuning the top octave for every different key I played in. I don't think other hammond models have stretch. I won't be buying any electronic tuners any time soon. I object to buying everything from a country that treats its workers like ****. Same problem with tuning wrenches. The name brand of the tuning wrench from 20 years ago doesn't tell you anything about its quality now. There are a lot of people who have gotten very rich by paying to have the appearance of quality products copied, by having them sourced from the land of imitation steel, imitation concrete, imitation milk, polluted city water, etc etc etc. If you can buy a genuine antique tuning wrench from a retiring pro, wonderful, but i wouldn't trust anything new for sale by mail order, internet, or phone order. The Sears wrench is not quite the tapered shape, but it is made out of real steel, and can be modified out to a taper with a real nicholson file if you're worried about it.