A few answers to OP's questions:

Could I spend a day with some instructions and get a half-decent job done?

Yes. Lots of people "touch up" sour notes on their pianos between regular tunings.

Or would I just damage the piano or make it worse?

That's a possibility. The main danger is that you break a string, but you can mostly avoid that if you are careful. If the piano is really out of tune, chances are you'll make the tuning better, not worse.

Good instructions (books or online) for learning how to tune a piano would also be appreciated.

When I first started tuning pianos, I tried using Wikipedia's page on Piano Tuning. While that is a valuable resource, it was not (and still is not) a good tutorial. Later I got the book Piano Servicing, Tuning, and Rebuilding by Arthur Reblitz, which was much better, with instructions and exercises for learning to tune "aurally" (by ear). Learning to tune by ear is a big commitment, so lets move onto online tutorials for tuning with electronic tuners. Here are some of the ones I've found:

• Wikihow - Tune a Piano - Let's get the obvious benchmark out of the way. This is a very basic tutorial that's extremely short on details. (It's like, "Tune middle A, then tune the A below it. Then use lower A to tune the note one fifth above, and so on, until the entire piano is tuned.")
• Detwiler tutorial - A much longer and more detailed tutorial that's been around for a long time. They advocate using an inexpensive Korg chromatic tuner for the middle octaves and then tuning the treble and bass by ear. (Much more accessible than trying to tune the middle of the piano by ear)
• Howard Piano Industries tutorial This is an 8-part video course with an accompanying PDF workbook. Probably the most detailed course you'll get for the money (free).
• Tune Your Own Piano A newer tutorial for DIYers. Also fairly detailed, but no videos. You can also find several non-free online tutorials and courses, and several decent videos on YouTube.

A few answers to OP's questions:

Could I spend a day with some instructions and get a half-decent job done?

Yes. Lots of people "touch up" sour notes on their pianos between regular tunings.

Or would I just damage the piano or make it worse?

That's a possibility. The main danger is that you break a string, but you can mostly avoid that if you are careful. If the piano is really out of tune, chances are you'll make the tuning better, not worse.

Good instructions (books or online) for learning how to tune a piano would also be appreciated.

When I first started tuning pianos, I tried using Wikipedia's page on Piano Tuning. While that is a valuable resource, it was not (and still is not) a good tutorial. Later I got the book Piano Servicing, Tuning, and Rebuilding by Arthur Reblitz, which was much better, with instructions and exercises for learning to tune "aurally" (by ear). Learning to tune by ear is a big commitment, so lets move onto online tutorials for tuning with electronic tuners. Here are some of the ones I've found:

• Wikihow - Tune a Piano - Let's get the obvious benchmark out of the way. This is a very basic tutorial that's extremely short on details. (It's like, "Tune middle A, then tune the A below it. Then use lower A to tune the note one fifth above, and so on, until the entire piano is tuned.")
• Detwiler tutorial - A much longer and more detailed tutorial that's been around for a long time. They advocate using an inexpensive Korg chromatic tuner for the middle octaves and then tuning the treble and bass by ear. (Much more accessible than trying to tune the middle of the piano by ear)
• Howard Piano Industries tutorial This is an 8-part video course with an accompanying PDF workbook. Probably the most detailed course you'll get for the money (free).
• Tune Your Own Piano A newer tutorial for DIYers. Also fairly detailed, but no videos.

You can also find several non-free online tutorials and courses, and several decent videos on YouTube.

A few answers to OP's questions:

Could I spend a day with some instructions and get a half-decent job done?

Yes. Lots of people "touch up" sour notes on their pianos between regular tunings.

Or would I just damage the piano or make it worse?

That's a possibility. The main danger is that you break a string, but you can mostly avoid that if you are careful. If the piano is really out of tune, chances are you'll make it better, not worse.

Good instructions (books or online) for learning how to tune a piano would also be appreciated.

When I first started tuning pianos, I tried using Wikipedia's page on Piano Tuning. While that is a valuable resource, it was not (and still is not) a good tutorial. Later I got the book "Piano Servicing, Tuning, and Rebuilding" by Arthur Reblitz, which was much better, with instructions and exercises for learning to tune "aurally" (by ear). Learning to tune by ear is a big commitment, so lets move onto online tutorials for tuning with electronic tuners. Here are some of the ones I've found:

• Wikihow - Tune a Piano - Let's get the obvious benchmark out of the way. This is a very basic tutorial that's extremely short on details. (It's like, "Tune middle A, then tune the A below it. Then use lower A to tune the note one fifth above, and so on, until the entire piano is tuned.")
• Detwiler tutorial - A much longer and more detailed tutorial that's been around for a long time. They advocate using an inexpensive Korg chromatic tuner for the middle octaves and then tuning the treble and bass by ear. (Much more accessible than trying to tune the middle of the piano by ear)
• Howard Piano Industries tutorial This is an 8-part video course with an accompanying PDF workbook. Probably the most detailed course you'll get for the money (free).
• Tune Your Own Piano A newer tutorial for DIYers. Also fairly detailed, but no videos. You can also find several non-free online tutorials and courses, and several decent videos on YouTube.

A few answers to OP's questions:

Could I spend a day with some instructions and get a half-decent job done?

Yes. Lots of people "touch up" sour notes on their pianos between regular tunings.

Or would I just damage the piano or make it worse?

That's a possibility. The main danger is that you break a string, but you can mostly avoid that if you are careful. If the piano is really out of tune, chances are you'll make the tuning better, not worse.

Good instructions (books or online) for learning how to tune a piano would also be appreciated.

When I first started tuning pianos, I tried using Wikipedia's page on Piano Tuning. While that is a valuable resource, it was not (and still is not) a good tutorial. Later I got the book Piano Servicing, Tuning, and Rebuilding by Arthur Reblitz, which was much better, with instructions and exercises for learning to tune "aurally" (by ear). Learning to tune by ear is a big commitment, so lets move onto online tutorials for tuning with electronic tuners. Here are some of the ones I've found:

• Wikihow - Tune a Piano - Let's get the obvious benchmark out of the way. This is a very basic tutorial that's extremely short on details. (It's like, "Tune middle A, then tune the A below it. Then use lower A to tune the note one fifth above, and so on, until the entire piano is tuned.")
• Detwiler tutorial - A much longer and more detailed tutorial that's been around for a long time. They advocate using an inexpensive Korg chromatic tuner for the middle octaves and then tuning the treble and bass by ear. (Much more accessible than trying to tune the middle of the piano by ear)
• Howard Piano Industries tutorial This is an 8-part video course with an accompanying PDF workbook. Probably the most detailed course you'll get for the money (free).
• Tune Your Own Piano A newer tutorial for DIYers. Also fairly detailed, but no videos. You can also find several non-free online tutorials and courses, and several decent videos on YouTube.