I’ve been practicing tongued octaves for a few days. It wasn’t easy for me at the beginnig. From the A above middle C I had quite a hard time getting the note right. I wasn’t able to find the right shape. That frustrated me a bit. Then the idea of the piano came to me. As simple as it is, it is also effective! Before I play an octave of a note above middle C, I play the desired note on the piano to hear it exactly in my ear. With this little help, I manage to find the tones more easily and to remember the corresponding shape better and better. Maybe it will help one of you out there too