My trainer and I have been working with my younger horse, who just turned three, teaching him what he needs to know before we back him for the first time. She also helped me start my first horse, who is now seven, so this is the second baby I have gone through the training process with.
A lot of riders don't realize how much work it takes to get a horse ready to be ridden. You don't just hop on and go -- or, as some cowboy types might think, hop on them and let them buck it out! As my husband likes to say (and he's not even a horse person), you wouldn't get into a car with no brakes and no steering wheel, especially not if that car was on a hill! Similarly, before you ever get on a horse's back, he needs to know and understand all the important commands you might need to give him from his back.
Obviously, the very first thing you ought to start with is ground manners. A horse needs to know it's not OK to treat people like other horses. This means being quiet and respectful and listening for what the person might ask for next and most importantly, no biting, kicking, pulling, crowding or any other dominant behavior a horse might offer to another horse in the pasture.
Ground manners ideally should be established from a young age. Next is lunging, which teaches a horse the different gaits. I don't mean that the horse doesn't know how to walk, trot or canter on his own -- obviously he does, but he needs to know which gait you want when you ask him to speed up or slow down. A horse needs to be able to differentiate between what you are asking, at a finer level than just "walk" or "RUN!"
The other important thing that the horse learns with lunging -- and should be a part of his ground manners, too, for that matter -- is the concept of the whoa. A horse's brakes are equally as important, if not more so, as his understanding of the commands for the different gaits.
And finally, a horse needs to know what the rein commands mean. Contrary to what some riders I know seem to think, horses are not born knowing that when you tug on the right side of their mouth, they should turn their whole bodies (and not just their heads) to the right. The concept of both reins for whoa is a whole new concept, too. This is typically taught before you ever get on the horse's back via ground driving, which is what my trainer is doing in the picture above. My horse has a lunge line attached to each side of the bit, just like reins; the lines then go through a D-ring on each side of the surcingle he is wearing around his girth, which ensures that the pressure from the "reins" is always coming from the horse's back, just like if there were a rider there. Teaching the horse to give to rein pressure in this manner ensures that if he freaks out, you are not only NOT on his back when he does so (remember the car on the hill without steering or brakes), but also a safe distance behind him (too far back to get kicked inadvertently).
Before I hired my trainer to help me start my first horse under saddle, I had no idea how much work goes into training a horse. Even once you have backed them, there is still lots of fine tuning to do, such as teaching them to bend properly and to move away from leg pressure!