I'm a lot autodidact, even though I learned basics at conservatory. I can tell that there are few most important things / axioms to keep at all times:
- Sound first: what matters the most is the sound you produce, you might innovate in the technique without hurting yourself; If you got the sound you want , that's it. After a while, you'll learn actually to adapt to your instrument and you'll learn from it. Each instrument has its possibilities in terms of sound, and you cannot ask the same thing to small pianos, small grand pianos, and Steinway D pianos. Even two same models should have different sounds, different mechanics, and you should adapt your technique to the piano you're using. That's also why I can tell which piano is a certain pianist working on: their technique is built from their piano, through hours and years of practice.
- Relaxation: another important fundamental thing in piano, especially when you have lots of notes, is the relaxation; once the note is played, the idea is to relax as soon as possible. One sensation I might recommend is to feel that tension is occurring only at the tip of your fingers. At some point, you might feel only a contact between your body and the chord, or your soul and the sound, if I may talk like this. I received this teaching very early, it comes from Fassina. Some study this in 3rd cycle / perfectionnement, but in that case, they need to remold a lot of their previous technique.
- I learned a lot with somatic education and contemporary dance; especially how to use my muscles and how to use my bones; anyone can balance any effort, any push between using more weight and bones (like Richter, and other pianists sitting a bit high) and using more muscle (you're still using bones and weight, but less, like Horowitz or Art Tatum, who was pure autodidact and develop its own movements and piano technique). I heard that the first technique is more French/German, and the latter Russian. I encourage you to try to feel those things, to watch videos of pianists that inspire you and see their position and body usage to make their sound. Later, you might go from technique to the other and have huge freedom like Keith Jarrett, standing, lying, etc. The seating height might matter, but the wrist height position is more important and helps you to switch from bone to muscle approach. usually a "school" teaches you one one them and they want to stick to that. But in the end this doesn't matter: you can choose your approach depending on the piano piece, and the sound you're looking for. Whatever the technique trend you're using, relaxation (and the ability to continue to play and to make the sound you want) is what matters: we come back to the first two points. This is not taught in music at all.
- Work with love, motivation, enthousiasm: pick the music you want to play and play it. It helps to discover / make your own technique and technique discoveries. This is something I needed to unlearn from classical music and learned from acting: get rid of fear of imperfection, you play as you are today, right now, and your music has all its beauty in the authenticity of your life.
I would like to mention few authors that are really helping to improve your technique:
- Bach music, multisounds, multivoices (fugues) at each hand is helping to improve your finger mobility and technique: because another finger is having some constraints, you are required to have hand and body relaxed and find the resources in the other finger to play what is needed. I love Bach toccata; Schuman toccata, Prokofiev Toccata, Chopin sixtes and third Etudes Liszt, etc are really teaching a lot, and especially Godowsky Etudes. Chopin was recommending to practice - play Bach everyday. The actual point is to play fugues or multisound regularly.
- Liszt's music is unique in teaching an important technique aspect: freedom in your arms and body. you'll find the right balance between muscle approach and bone approach, between tension and relaxation. Many pieces are not that difficult and can be given to beginner-medium, but it's a huge step to play aerial pieces (with jumps, hand moving a lot in registries, eg., in Etudes d'execution Transcendante, 3,4,5,7,8,10,12).
- Ultimately, working your left hand will improve everything, and using unusual fingering will help you reach an amazing technique. This is more something pianists are starting to study seriously after they complete conservatory cycle, but I think this can be started since the beginning. The best cycle to develop this is the Godowsky Etudes, after Chopin Etudes. Many can tell you that it's extremely difficult, maybe, but it can also be approached early.
One piece of advice about work rhythm: it's really helping with motivation and drive to complete a piece in 1-2 month. You might come back later on it but spending too much time on something may drive you into wrong technique, tensions, etc. I was given an amazing piece of advice: to mix pieces with different techniques.
Another fundamental point learning rhythm: The best situation is to alternate slow work for details and regular speed playing. The latter helps you to feel and learn the right movement. Many pianists are blocked in their progression because they spent too much time in the low speed learning part: they learned things but it's not the right movements. So they spend days to unlearn what they did in the slow approach. What matters is to feel that you have the freedom to play in a way or another; this is the most time consuming part of learning, but most pianists don't do that. Argerich feels blocked in her piano, I think because of that. Repetition is not learning. Horowitz never played a piece twice the same way. Very few are doing that. The others are executing what they programmed themselve to do. You can put your soul and build a universe in few seconds of music.
At last, Art Tatum, Cziffra were huge autodidacts, and their way to use their body and piano is unique. Horowitz is also quite unique. I learnt mostly from him and Cziffra, and a bit from Richter. Everything Tatum is doing makes sense to me, but he really lack thumb/wrist technique, which is the opposite of Cziffra, who tried to use the thumb like another finger. Youtube is your best friend. Richter's first piano piece he played was a Chopin Nocturne ! Go for it, write a jazz version, compose and improvise ! He (and I and Argerich) never played scales or boring things. Music first, enthousiasm first !
Good luck and enjoy !