It's not hard to do...it just hard WORK
Years ago, our old English farrier (who was 75 years old and still working), convinced me that I could do it myself. After some practice, advice, and a few visits to check my work, I've been doing our mares for years.
The more you ride, the less work you have, and I only need to use a rasp to keep them trim, mainly cleaning up uneven wear and keeping the toe short and rolled over.
I'm 54 yrs old (and tall), and you do feel it in your back and arms, so I usually only do one mare a weekend and have a big bottle of ibuprofen nearby. I've found that doing it this way (each mare every 3 weeks) keeps them nice and trim and easy to do with only a rasp. It's more work in the summer when their hooves are growing more, less work in the winter.
Not only does it save $$s, but you learn a lot about how each horse wears their hoofs, and they become very used to having their feet handled...I can go out and trim them without even having to tie them.
They're all different, but my experience has been that (with riding), most wear their heels pretty much naturally, and you usually just need to keep the toes short and rolled over. (One of our mares, though, does have very tough hooves and will get long in the heel..she requires the most effort)
My biggest learning challange was being shy about trimming the toe too much (or ruining their feet), and I would tend to leave them a little long. I think this goes along with comments about understanding the angles...the hoof looks a lot different when you're looking at the bottom with a rasp in your hand than it does when you see the horse standing, but one advantage of doing it yourself is that you can always go back the next day or week and trim them a little more until you get the feel for the right amount. Also, I rarely use a hoof knife unless I want to clean up the sole more since they will wear fine by themselves and really toughen up...I've found that many farriers take too much off the sole leaving them ouchy.
Others have posted good references to review in addition to getting advice from your trimmer/farrier. Keep in mind that each trimmer/farrier has their own thoughts and style, and just like training, there is probably no one right way.
Good luck, it's well worth it, and on a hot summer day, you're really appreciate the amount of work a farrier/trimmer does to make a living.