Buy a textbook. Read it, thoroughly. I started with Pete Ramey's book: http://www.amazon.com/Care-Rehabilitation-Equine-Foot-Ramey/dp/0615524532
I took an "Intro to Farriery" course at the big university that has equine studies. I learned all about shoes, shoeing, trims, etc. I watched a farrier custom make 4 shoes, hot fitted them to a horse and nail them on - all in an hour. I watched him make MANY shoes over the course of class, but this was more for my own education. I don't know how commonplace a class like that is. He didn't go over barefoot trimming very much.
Then I approached my farrier and asked if she would be willing to help me - she agreed! I started looking at hooves, touching hooves, really watching how horses move. I started holding horses at my barn (much to my BO's delight since it freed her up!) and asking questions as my farrier worked.
I am more concerned about MY horse, so we took more time with him. At some point I think she felt I was getting educated, so she started 'testing' me. We would look at my horse's hooves one at a time and I had to tell her my thoughts. She'd correct me, expand on things, and then she'd do the trim. At this point, I am 'in charge' of one of his hooves and she does the other three. The one hoof I am responsible for doing when she's there. She hasn't corrected me on his hoof in the past 2 trimmings and on the next, I am moving on to two hooves.
I'm not looking to trim for money - I just want to be able to manage my horse. I'd still have her look at him every so often though once I'm up to all four hooves on my own. I still help with the other horses and ask questions.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.