Practice playing chromatic scales with just your pinky and then just your thumb, then put them together.
How to Play Octaves with one Hand by: Ido Ronen from www.piano-play-it.com
Hi Emma,
Octaves are used frequently when playing piano. Your question is excellent and I hope my answer will help you and other students who encounter the same problem.
Here are two exercises that will help you to improve your playing octaves:
How should these exercises be practiced?
1. In both exercises play the right hand, then the left, and when you feel you're ready you can play the exercise with both hands.
2. Play very slowly at the beginning. Gradually increase the tempo, but remember that playing fast is not the main issue, so be careful when you speed up.
Now, a few comments for each of the exercises:
The first exercise helps to locate the pinky and the thumb on the correct keys. This exercise should be played upwards along the scale (as written) and downwards as well (C-C, B-B, A-A...).
The second exercise focuses on moving from one hand position to the other. There are only four measures written above; you should continue enlarging the skips until you get a skip of a whole octave. C-F-C, C-F#-C, C-G-C... C-B-C, C-C-C).
These exercises can help you to overcome the difficulty you're experiencing. If you practice with patience, I'm sure will develop this skill.