The key to muscle memory is to create an automatic pathway traveling down the nerves from the brain to the fingers. The movement must be done accurately over and over. To ensure accuracy, the movement must be broken up into small bits and done extremely slowly. After the movement can be done slowly "without thought", then the speed can be increased incrementally. Increase the tempo by tiny amounts at a time and only increase as the accuracy remains. As soon as a mistake is made, you know you're going too fast. Slow back down a little and do it again. Don't expect this to happen in one practice session. Build up over time.
As Jomiddnz suggested, start with one key. Get used to where the buttons are located in reference to each other. When playing slowly, take the time to really feel the distance. How much do your hands stretch? How far apart are your fingers? Do you move up to the right or down to the left (or whatever the movements are for accordion.) The idea is to be mindful about every motion.
I use this technique when practicing piano and even used it today when I was getting everything right except for stumbling in one or two measures. I slowed those measures right down and got to work. I am an excellent sight-reader, and the reason is because of muscle memory. I am terrible at memorizing songs, but I have the piano keyboard memorized. I can play it blindfolded if I know the song because I can feel where every single key is in relation to my body. So when I am sight-reading a piece, my mind can translate those notes into hand and finger positions on the keyboard. It really works, but there's not shortcut.