Tibetan #4

Hi there Have recently started to practice the Five Tibetans and getting enjoyment and energy from them. The exercise I have most difficulty with, and the one that I sometimes fear I could be causing damage to myself is #4. My feet do slip (they're not suppose to) and in general it's a toughie. Could it be just that I have started straight in by doing 11 repetitions? Please advise if possible and let me know what I should look out for as a "danger sign". Thanks Luis Tweet