The problem with playing an instrument is translating the static information on the page into small and intensely specific movements. In essence you are issuing instructions to your body.
In the early stages of learning something, I find that it helps if I can find a form of words which will do this translation for me. I can only write about what I know, so I have chosen examples from teaching the piano.
I tell my students that they have to “give orders” to their fingers, like training a dog to sit or lie down.
Take the scale of C major on the piano; the right hand fingering for two octaves is
123 1234 123 12345
If you teach the student to make sure that they count their fingers up and down, they will get it right, and avoid the mistake of starting out “12345oops”. Count with them, then ask them to count for themselves, and then to count silently. Soon it will become automatic.
Now take the scale of C major, hands together;
I recite the instructions;
“1, 2, DOUBLE 3, (said with Great emphasis) first thumb, second thumb, DOUBLE 3, FOUR (meaning RH, or they may play their thumb),
DOUBLE THUMBS, 4-over, DOUBLE 3, first thumb, second thumb, DOUBLE 3, FOUR (meaning RH) and you’re there!
Now we need to get down again;
“a finger, DOUBLE 3, first thumb, second thumb, DOUBLE 3, FOUR (meaning left hand; otherwise they will play their LH thumb),
DOUBLE THUMBS, 4-over, DOUBLE 3, first thumb, second thumb, DOUBLE 3, next finger, and you’re there!
F major, hands together is a little different (and that’s why I always leave it until last in the early grades) as the thumbs always coincide
1234 Thumbs-3over,then Thumbs-4over,then Thumbs-3over, ending on a 4 (meaning RH, which reminds us to put 4 on the flat coming down
Off we go, then Thumbs-4-over-on-the-flat, THUMBS-3-over (said with emphasis, as they will try and play LH thumb on G), Thumbs-4-over-on-the-flat and you are home.
These sets of “instructions to self” are too complicated to remember and internalise all at once, so it is important to break it up into small steps. Say one octave going up, two octaves going up, and eventually building up the mental stamina to do the whole scale.
In the context of learning pieces, I will find forms or words to explain to my fingers how they interact with each other; “thumbs next door to each other” or give chords names; sometimes they are proper names like “Cminor-2nd-inversion” which is the first chord of the Chopin prelude below, and other times, “black-note-splatty-thing” which is the seventh beat (I have small hands and have to spread most of the chords, or omit the bottom note).
I have used the same kind of verbal instructions to persuade my fingers to travel to the correct places when playing other instruments. Examples are learning shifts and finger patterns on the cello, guitar, and ukulele, or fingerings for chromatic notes on recorders, clarinet and saxophone. I unexpectedly found myself in the position of learning the trumpet earlier this year (that’s Wider Opportunities teaching for you!) and again, created mental “instructions to self” to remember the sequence for pressing down the valves. Unfortunately I could never be sure which note, if any, would eventually emerge!