For anyone who wants to learn, I can very much understand being nervous at first. Harvey and I both were, but we were lucky and our farrier wanted to help us learn to do our own horses and was a great mentor.
It took awhile before we felt confident, but like I said, we've been doing it ourselves for a few years now and on all age groups. The horses have stayed "straight" and never been lame. We each look at the foot and just take it slow every time. I really prefer that we do them ourselves at this stage because after we've finished trimming a horse, they look exactly like I want them to -- which is exactly how they should look.
Those of you "tempted" may be surprised that your farrier may be very pleased to help teach you. Our farrier was (is) very good and had plenty of clients. While he loved our horses and ours behaved well for him, they are back busters and he was not sorry to help us learn to do the job ourselves.
Another piece of advice I can give is to not go with "cheap" under $20 nippers. Go for the more expensive $80+ kind as they work MUCH better. Also, go with the smaller "mini" rasps.