It can take several trimming sessions to get feet right. Bear in mind what a "perfect" foot should look like and rest the bottom blade of the footshears flat on the base of the foot, trimming from back to front. That way you won't take off too much. If it bleeds or you're shaving off more than a small sliver of white line, you're going too deep. No sheep should be lame after trimming if it wasn't before. Yes, you'll probably have some where the hoof sticks out at the front or sides but don't be tempted to make them look pretty! They will grow out, given time and some dry weather and you'll be able to get them right.
As an aside, when I have folks choose sheep on the farm for collection later I always get them to spray marker on the backs, so they know that what they chose is what they're getting.