HOW: Get set up in a seated position barefoot with your foot resting on the ground. While maintaining good arch position and keeping your heel and the rest of your foot relaxed on the ground, begin the exercise by lifting your big toe in isolation while the smaller toes are relaxed, then lower the big toe and lift the smaller toes with your big toe relaxed. Repeat.
FEEL: You should feel your foot muscles working to perform this exercise and control the motion. It may be challenging and even frustrating at first, but try to relax your foot and 'try less' versus tensing up.
COMPENSATION: Avoid the urge to excessively flex and curl the big toe / little toes when they're resting on the ground, just use a little pressure. Try to keep the rest of your leg above your foot/ankle 'quiet' and relaxed.