Easy... just watch and repeat. 25x each foot. The pictures showes a variation from the video, they are both correct and good exercises.
Once this is easy, reduce the speed. http://www.youtube.com/watch?v=sTM2TXQrMxA
Then you will have a super strong tibialis posterior !
Here is a little more information about the tibialis posterior, to understand its importance
The tibialis posterior is the most central of all the leg muscles, and is located in the posterior compartment of the leg. It is the key stabilizing muscle of the lower leg. Blood is supplied to the muscle by the posterior tibial artery, and innervation is via the tibial nerve.
Origin and insertion
The tibialis posterior muscle originates on the inner posterior borders of the tibia and fibula. It is also attached to the interosseous membrane, which attaches to the tibia and fibula. The tendon of tibialis posterior muscle descends posterior to the medial malleolus and terminates by dividing into plantar, main, and recurrent components. The plantar portion inserts into the bases of the second, third and fourth metatarsals, the second and third cuneiforms and the cuboid. The main portion inserts into the tuberosity of the navicular and the plantar surface of the first cuneiform. The recurrent portion inserts into the sustentaculum tali of the calcaneus.
Function
As well as being a key muscle and tendon for stabilization, the tibialis posterior also contracts to produce inversion and assists in the plantar flexion of the foot at the ankle. The tibialis posterior has a major role in supporting the medial arch of the foot. Dysfunction of the tibialis posterior, including rupture of the tibialis posterior tendon, can lead to flat feet in adults, as well as a valgus deformity due to unopposed eversion when inversion is lost.
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