Recently, my colleague Dick Hamlet did an amazingly generous thing. He gave me a beautiful old upright piano. He was moving, and didn't want to move it, so I had it hauled to my house. How cool is that?
The piano needed some work, so Keith Packard and Carl Worth did another amazingly generous thing. They came over and helped me fix the piano. Then Keith tuned it. This in itself took several hours.
My Mom used to tune our piano when I was a kid. Since then, I'd never seen it done. So I thought maybe some of you would be interested in how Keith did it. It's quite repeatable, actually; all the way to easy. It just takes time, patience, quiet, and a bit of equipment...
DISCLAIMER: Why would you follow instructions by some idiot like me? I obviously have no idea what I'm talking about. When (not if) you break your piano, it's your own darn fault. When (not if) you hurt or kill yourself, don't cry to me about it. Piano strings are under tremendous tension; this makes them potentially lethal. Further, pianos are heavy; they can kill you just by falling on you. You have been warned.
Still here? First, purchase a tuning kit, available from any good piano store. The kit should contain a long-handled wrench for turning the tuning pegs, a bunch of felt and rubber tools for damping strings, and one or more tuning forks. Ignore the tuning forks; they're just misleading you.
Next, get Keithp's tone generator code, and compile it against fltk for your Linux laptop. This gives you a nice stable tone generator that can generate any note of the main octave, in a variety of tunings. Make sure you select "equal temperament", because that's how pianos are supposed to be tuned.
Now, take the piano apart until the strings are unobstructed by covers, dampers, etc. Do not mess with the hammers, felts, action, keys, etc. Anything that looks delicate is; you might do big damage.
Press the middle C key on the piano gently, and notice the hammer moving forward to strike the strings. It, like all notes in the main octave, will be aimed at a particular group of 3 strings. These 3 strings are supposed to be at the same frequency—multiple strings makes the note louder. Take the felt strip from your tuning kit, and press a fold of it into the gap just to the left of the group. The rubber tool from your tuning kit can be helpful for this. Then loop the felt strip up over the group, and push another fold of it into the gap to the right. Note that now only the middle string of the middle C group can vibrate. This is our goal. Keep moving to the right and pushing folds of felt into gaps between groups until you get to the C above middle C. Then stop. Now the central octave of the piano has only one note playing per string when a key is pressed.
Set the tone generator to play C, then put the wrench on the pin that tunes the middle string of middle C. Strike the middle C key, and listen for the difference between that and the tone generator. If the piano is badly out of tune, it will be obvious; this is unlikely. More likely, what you're listening for is "beats". A beat between two tones makes them sound like they're getting louder and softer. Maybe a few times a second, maybe ten or twenty times a second. Thrum-thrum-thrum. It's caused by interference between the tone generator and the string; it's hard to describe, but once you've heard it it's unmistakable. The goal is to tune the string to get rid of the beats.
Put ever so slight pressure on the end of the wrench handle. Probably you'll need to move it clockwise, to incrase the pitch of the string. As you start to tune the pin, you may feel the pin itself flex a bit, then the pin push back against the wrench as it moves. This is normal. At some point, the beat frequency should change slightly—the beats will start happening faster or slower. If they're faster, you're going the wrong way. If they're slower, you're going the right way. Relax the wrench and see how close you are. Repeat until the beats are gone. Now you've tuned one string on the piano. Repeat until you've tuned the center string on all 13 notes in the main octave.
Be sure to use small, gentle, minimal pin movements in tuning. Massive overtightening of a string might ruin the instrument. Failing that, the more tuning you do, the quicker the piano will drift out of tune. Minimal is best.
Once you've tuned the center strings of the main octave, you're done with the tone generator: set it aside. Now, pull out the leftmost fold in the felt strip you inserted earlier, which will let the left string of the middle C vibrate freely. Tune the left string to the center one by nulling out the beats, the same way you did with the center string and the tone generator. Pull out the next fold of felt, and tune the right string to match the other two. Now you've tuned one note, and are ready to tune the left string of C#.
When the main octave is tuned, move the felt strip up or down an octave, then beat tune each center string in the new octave to the string in the main octave. Continue in the obvious way until the whole new octave is in tune, then keep going.
The highest and lowest notes on the piano do not have 3 strings per note. For these, the rubber tool can be used to damp one of two notes to tune the other string in that group. When you're done tuning, put the piano back together and put the tuning kit somewhere safe for next time.
That's pretty much the story. Getting someone who's done it before to help you the first time you try is recommended. Have fun!