1. Freelunging/lunging on a line starts as a yearling, focusing more on freelunging as I don't feel lunging on a line is good for young legs/necks. However, my yearlings are taught the basics of a lunge line so that when we go to Nationals or a show, I can get the sillies out in a controlled manner as lots of facilities don't have turnout. They learn the verbal cues of walk, trot, reverse, whoa, and come. It's mostly like roundpenning, using their natural behavior to eventually understand what the cues mean.
2. At age two (usually early spring, like February since we have mild winters here) they start wearing a bridle and surcingle during these workouts. They wear the bridle while I clean their stalls and groom them. I use fruit snacks ("fruit roll ups") on the bit to make it a pleasant thing, and eventually use it less frequently so they are not hard to bridle). When their mouth grows very quiet to the bit, I start introducing sidereins very loosely and eventually adding a check rein until they are bitted up in a decent frame (takes a few months normally). They still have the freelunging/lunging on a line routine, just now they have to wear "clothes" and eventually work their neck and back in the frame I need (I do breed shows, so this won't work as well for carriage type driving if that makes any sense. Forced frames are not desired in that case!).
3. Summertime hits and it's time for ground driving in the roundpen and arena. They learn to set their head without the side reins (I use gentle massaging to encourage this) and how to turn with the bit. At this time, they are also integrating to the blindered/closed bridle. When they are very responsive to the ground driving, they graduate to ground driving outside around the neighborhood (since weanlings, they go for hand walks around the neighborhood to see all sorts of scary things.)
4. Late fall of two year old year, I consider hooking them. This depends on a few things. A. They need to be able to stand still at a "whoa" as long as I want, no matter where they are or what is going on. I hook my cart in the open (not tied) by myself and I expect the horse to stand still for hooking. B. They haven't shown any dangerous propensities such as bolting or shutting down to cues when frightened. C. They do not back up continually when scared D. They turn both ways equally well. When these things are met I will start ground driving them with one hand and holding the shaft of the cart with another so they can hear it behind them. A helper is better for this if you can get one! Then after a few days of that, I will practice hooking them, leading them for about two laps around the arena, and then unhooking them. If this goes well, I will begin to ground drive them with the cart so that they can slowly get used to the shafts, as usually turning is a bit bewildering to them when they feel the shafts. Meanwhile, this is integrated with regular bitting up as well as ground driving. I make sure all the above steps are always done to reinforce basics and prevent the horse from anticipating or getting bored. I still let them freelunge with nothing on, or putting them on a line, or something different.
5. When horse is quietly ground driving with the cart on, and standing still for hooking/unhooking, I will get in the cart. Usually by this step I don't have any problems still. I will usually keep the drives to about 5-10 minutes at the walk, and then unhook. After a couple times, we get to the trot. I always try to make it pleasant for them. I want my horses to be confident and comfortable and enjoy the exercise.
At any time, if a horse is frustrated or doesn't "get" something or I'm having difficulty, I just stall and go back to the previous step until the horse is fine. I've had a couple of minis unable to progress past the "ground drive around the neighborhood calmly" step so I never got to hook them. Usually with patience and CONSISTENCY (working them a day or two a week, or a few times a month, is NOT progress that is good for the horse) you can get them hooked in a timely manner. But only the horse can tell you that!
Some horses require you to try something else, as not all horses think and learn the same.
Andrea