I always make sure the fondant is nice and tight to the bottom of the cake and then use a pizza cutter and roll it around the cake at a 45 degree angle. If I get in a rush and don’t get that fondant right up against the cake all the way down to the bottom, then some times it will turn up a little short in spots.
As for tearing, is it possible you need to roll your fondant slightly thicker?
Hope that helps. Keep at it, you’ll get it!
