On the first you just cross your arms above the bar and then hold on by pressing against the bar with the opposite hand over the opposite shoulder. On the second just clasp your hands together and raise them up to the front of the bar and push against it. Both of these methods of holding a front squat should be last resorts, as should that contraption called the front squat harness. Try to stay away from crutches. When doing front squats sets and reps are up to the individual.
Higher reps are tougher, but they can be very rewarding.
I found I made my best gains on front squats by doing 4 sets of 8 to 10 reps.