Good advice above.
Start slowly and build up.
1. Leading - horse moves forward when you do and halts when you do. Horse can be turned right or left by handler.
2. Ties. Horse stands quietly and doesn't pull back when tied.
3. Grooming. Horse can be groomed, without a fuss, all over it's body - from inside it's ears to cleaning it's hooves and sheath/udder.
4. Horse stands quietly for farrier and doesn't lean.
5. Horse stands quietly for bathing, including sheath.
6. Clipping. Horse stands quietly for having it's legs, face and ears clipped.
Now for the fun part.
7. Start teaching now leg yield (from the ground) - make certain you teach it on BOTH sides.
8. Turn on forehand, turn on haunches, Rein back
9. Lunging - NOT tight circles. Nor very long (10 minutes max to start). Teach horse voice commands starting with walk on and halt. Over time add trot and eventually canter.
10. Carrott stretches - between front legs, to left and right to middle of the back (top).
11. Saddle pad/ blanket - acceptance of strange thing on it's back.
12. At 1 1/2 years you can place a LIGHT saddle on their back (No girth/cinch). If they handled the saddle pad the girth shouldn't be a big deal.
13. Have someone stand on a mounting block, sit on top of a stall, etc...somewhere where the rider is ABOVE the horses head/back. (Some horses freak when they see a human "above" them.)
14. Pony the horse (ride another horse with young horse on the right side, horse you ride should be VERY quiet and controllable with one hand). Start in the pasture where if horse gets loose it shouldn't be a big deal. Make certain you can turn in BOTH directions, halt and rein back. Once you've got the walk try a trot.
15. Before you teach horse to accept bit you need to have an equine dentist (or vet) look at it's mouth. Between ages 2-5 years the horses looses it's "baby" teeth and thus a bit in the mouth can be painful IF the baby teeth (caps) are coming off or the horse has some other sort of problem. Not all vets are good at this, and neither are all dentists, so choose carefully. (Mine happens to be a vet who does ONLY equine dentistry.) Teach horse about opening mouth for bit and accepting bit (I feed with bit in the mouth a few times).
16. Now add girth of saddle and lunging in tack (saddle/bridle).
17. Ground driving - walk, turn, halt and some trot whe horse does the rest perfectly.
By this time horse is around 2-2 1/2 years of age and you might be able to start riding. Teach walk, halt and turn....go slowly. Once you've got that down perfectly add in trot. Wait to canter until you have control of the trot.
Dressage is for Trainers!