The Miles method for learning music

• AKA, essentially the fast version of how I accidentally taught myself to do music.
• It's fun because, for most of it, almost right from the start, you're playing your favorite tunes, and for the most part you learn by playing. There's no studying and there are minimal pre-playing steps. And for the non-instrument parts (like whistling and finger-tapping), you can do them passively while you're doing other things.

Pitch & Melody

• Learn to whistle; do this until you are comfortable with it.
• Practice hearing a tone and whistling the tone you just heard; do this until you are good at it. There are apps that can tell you what tone you are whistling and if that matches the tone you are trying to match.
• Whistle along to all your favorite tunes, making sure to whistle the same notes as you are hearing.
• Whistle your favorite tunes from memory, making sure to listen to yourself as you do it.
• Congratulations, you have now trained your ear!
• Repeat this with humming, a kazoo, or singing to train your vocal pitch skills. This will get you 80% of what you need to sing well, because it'll have you singing the correct pitches for everything.

Rhythm

• Listen to various click tracks (for different time signatures), and finger-tap along with them, making sure to tap in perfect synchrony. (If you want, use your non-dominant hand for the big beats and your dominant hand's fingers for the smaller beats.)
• Try continuing to tap the click tracks after they stop.
• Finger-tap alongside the beats you hear in your favorite songs, making sure to match what you're hearing either the drummer do or what you're hearing in terms of rhythm from the other instruments.
• Finger-tap the beats of your favorite songs from memoryl
• Congratulations, you've trained your sense of rhythm!

Integration

• Whistle and finger-tap at the same time until you're comfortable doing so.
• Play around with stuff and put your own spins on things.
• Try improvising your own tunes and beats, just for funsies. Just do whatever comes to mind, and mentally note what sounds good and what doesn't.

Instruments

• Buy musicians' earplugs and wear them when you play; most instruments damage your ears with repeat exposure, especially indoors. Don't ruin your ears by playing music; it defeats the whole point.
• Pick an instrument you'd like to learn. (If you aren't sure, pick the tin whistle; it's one of the easiest instruments to get started on, it's cheap, it's portable, you can play it in sessions, and you can always learn other instruments later.)
• Make sure you finger it correctly and have correct posture. (If you don't, you'll learn bad habits that you effectively won't be able to unlearn.)
• Learn to play the instrument's native scale. Practice that scale until it's easy for you.
• Figure out how to play one of the simpler tunes you learned to whistle on your instrument.
• Figure out how to play more whistling tunes on your instrument.
• If applicable, learn to play the instrument's chromatic scale. Practice it until it's easy for you.
• Try playing your favorite tunes in their actual keys. The easiest way to do this, is to try to play the tune while you listen to it in slow-mo. Your ear should be sufficiently trained at this point for you to be able to hear whether you're playing the right notes. (You'll have to use chromatic notes to play in keys other than your instrument's native key. Playing non-native keys will rapidly increase your skill at your instrument.)
• Practice improvisation, like in the "Integration" section.

Whereto?

• Look for sessions where you live. There will almost always be an Irish session, but you may also find Old-Time, Bluegrass, Pub Sing-a-longs, or even some niche ones, like Quebecois, Scottish, Scandinavian, or Medieval. Most of these are not organized around sheet music, so all you have to do is learn a few tunes in the right keys, show up, play them, and (optionally) drink a (usually) free beer. TheSession.org is a really great resource for sessions (mostly Irish), and has a worldwide group lookup and sheet music with speed-controllable midi synthesizers. Sessions are the main social outlet for ear musicians.
• There may be instrument-specific groups where you live. These are more-likely to require you to also learn sheet music.
• Play music at your church, if you attend one. You may not be able to do complex Classical music without sheet literacy, but most hymns and liturgical music are extremely easy to learn by ear, and many smaller churches are lacking in musicianship and would be thrilled to have anyone performing.
• Show up at old folks' homes and play music for them; they will fucking love you.
• Show up at SCA events and play English country dances for people while they dance.
• Jam with friends and family.
• Start/join a band.
• Busk.
• Just play for fun.
• Play music in MMORPGs with keyboard-controllable instruments, like Lord of the Rings Online and Guild Wars 2. No-one expects to see someone performing, and people will appreciate the surprise.

Notes

• You will find that, once you know a tune in your head, you can immediately play it 80% correctly on your first try on any instrument you are skilled at. The other 20% is in ironing out kinks and acquiring muscle memory.
• You will be able to play any tune you want in any key for free, forever. Ear-playing means you never have to transpose notation; you just play with the fingerings for a different key, or you just sing at different pitches, or cetera.
• This method of learning music does not teach sheet music; you really don't need to know how to read unless you're trying to play in an orchestra or you want to learn how to play really complex Classical stuff and such. Learning music sheet-first (aka, how music is frequently taught nowadays) takes longer, is less-fun, and actually prevents you from training your ear and your innate senses of music (I know professional orchestra players who are worse at this stuff than I am...), and it locks you into spending a ton of money on sheet music for things you could easily have learned for free by ear. If you have a specific reason to learn sheet music, then by all means, do; but learn to play the old-fashioned way (by ear) first so that notation doesn't become an inescapable crutch.
• You technically don't even need to know what A-F mean, or what letter you are playing; but it's helpful to learn this, as it enables communication with others. You definitely need to know what "sharp" and "flat" mean, though.
• Tin whistle is a better first instrument than recorder, which has complex (fully chromatic!) fingerings and trickier higher octaves. (Schools chose poorly.)