Left Foot Overpronates

I noticed today that my left foot overpronates. If what I read is correct, this pretty much throws the rest of my body off alignment (since my left leg will turn and the rest of my body will shift), so I’m interested in fixing this problem ASAP.

My understanding of the problem is as follows (correct me if I’m wrong): when you take a step, the arch of your foot flattens out to absorb the shock, and the ankle rolls in (the foot everts). Now, if the arch of your foot doesnt recoil properly, then even when you’re just standing, the ankle rolls in and you’re standing on the inside of your foot.

So the obvious solution seems to be to strenthen the muscles responsible for the arch. However, lately I’ve been trying to increase my ankle mobility (no suprise that I have slightly more mobility in the right than the left), and part of the advice given for improving ankle mobility is to use a tennis ball on the bottom of the foot. This gets the muscles to relax and be more pliable.

What confuses me a bit is this: if the muscles of the foot relaxing results in overpronation, how is it a good thing? Should I skip the tennis ball till the overpronation problem is taken care of, or both use the tennis ball AND do exercises to strengthen the arch?

Do any of you have suggestions for correcting asymmetrical overpronation problems without external devices?

Sorry for the long post.