The Foot Corrector exercises are generally done in a standing position, one foot at a time, but they may also be done sitting. Feet and ankles should be warm and limber before using the Foot Corrector, so it is best to do your Foot Corrector exercises near or at the end of your Pilates session.
When performing the Foot Corrector exercises, as with exercises on any of the Pilates apparatus, you work the springs, they don't work you! This can be challenging because the exercises are done standing up with the foot on top of the pedal. It's important to imagine the foot working into the pedal, rather than just on top of it. And of course, scoop your abdominals in and up -- away from the downward motion of the foot.
The springs on either side of the pedal may reveal if the inner edge or the outer edge of the foot is stepping harder or sooner. Correcting imbalances like this can help with knee problems and aid in overall stability of the whole body.
Do all the exercises with one foot first, then stand up on both legs and take time to feel the difference between them. It is likely that the leg you just worked will feel longer, as if it is almost "hanging" from your body. The other leg may feel more "bunched up" at the top -- thicker and shorter. You may realize that both legs felt like this before the exercises and you weren't even aware of it!
As with all Pilates apparatus, correct usage is key. Be sure to learn how to properly work with the Foot Corrector -- it is well worth the effort. And, as simple as it looks, it's actually quite a challenge!