Do you have a wobble board? You need one! They are relatively inexpensive but extremely useful! You will start to see your strength and balance improve immediately!
This is an intermediate exercise, and I would recommend this one if you are already en pointe, and working to increase your intrinsic muscle strength, or get a higher arch.
A few key notes:
absolutely do not do more than you can do correctly! It is really easy to overwork yourself while using a wobble board, just like it's easy to overwork yourself in your new pointe shoes. Watch for "chunking," shaking, or major wobbles in your ankle while doing this exercise.
I always recommend doing extra on your weaker foot. This just helps even out the strengthening.
It may take some time to figure out your ankle or foot positioning on the board - be patient with yourself!