The above picture shows the position to start in. Firstly, tie a resistance band around a table leg, bed post, or some similar (and stable) structure. A red, medium-resistance band is best, but if you really struggle with this, then you can switch to a lighter band, such as a yellow one until that seems easy and then switch to a medium one. Sitting on a chair, put your foot in the loop made with the resistance band and place the band across your forefoot as shown. The outside edge of your foot should be on the side of the table leg for this exercise. Keeping your leg straight, position yourself so that the band is tight, but not being stretched much or at all. You are now ready to begin strengthening the Tibialis Posterior muscle.
Now, stretch the band by moving only your foot toward the midline of your body, away from the table leg. Do slowly and steadily and once you have stretched it as far as you can, slowly move back to the central position, and then slowly repeat. Do 10 continuous movements. You can do 1-3 sets of 10. Always exercises both sides unless you have been instructed to do one side only. Do this once each day. Never do more than you can do comfortably. You need to push yourself a little, but if you can only manage 5 or 6 reps, then start there and gradually increase this as your capacity improves.
An importance point to note here: only your foot should move. If you are rolling your whole leg to perform the movement, then you are using several other muscles, including your quadriceps, rather than isolating the tibialis posterior.