The best way to learn is to from a good, experienced farrier. Are you interest in trimming your own horse or others? If your own, I've found that good farriers are more than willing to spend time with you, teach you everything you need to know to start trimming your own horse, willing to check your work, give you feedback, and answer questions.
Years ago, our wonderful, old (70+) English farrier (50+ years experience) was trimming our mares and told me that I could easily do it myself. I was a bit concerned that I would mess up our mares' feet at first, but he taught me the basics, spent time with me, and felt comfortable that I was doing a good job. I've been trimming them for many years now, this has saved us a ton of $$s, and our mares are so used to me handling their feet that I can trim them without even being tied (They have to be patient since I'm not as fast as a pro). I would, however, still call a pro if I had a concern or they needed some sort of corrective trimming, since I have no experience with 'problem' feet.