Hi Luis! Well I'm no expert I'm not even a yoga teacher but perhaps you could help your lower back problems by practicing some "traditional" (ie. Indian or hatha) yoga, there must be classes available somewhere near you since there are classes everywhere these times. As for your feet slipping I agree with the other posts it is probably due to using the wrong surface. Try starting with just 3 repetitions and working your way up slowly, as other people said.