If this is the first time you will be saddle training a horse, it will be a good idea to hire a professional trainer to assist you.
Before a horse can be saddle trained, it should be able to lead, back away, move away from pressure and trot properly balanced in a halter or lungeing cavesson. Moreover, your horse should accept the bit and wear the bridle without resisting too much. Train your horse accordingly before moving on to the next step.
Take the horse to a fenced arena and lunge it. Ask the trainer you hired to hold the lungeline and approach the horse you are training with a saddle pad in your hand. Do not try to force the saddle pad onto the horse. Instead, familiarise your horse with the saddle pad and then settle the saddle into position. The horse might get spooked a few times but with successive tries, it will calm down.
Carefully place the saddle on top of the saddle pad. After the saddle comes the girth. Be sure to slowly adjust the girth to the appropriate tightness.
Give enough time to the horse to get familiar with the saddle pad and saddle. Attach the lunge line to the bit and remove the throatlatch for safety purposes. Wait until the horse you are training is familiar with the weight of the extra equipment you are adding.
Familiarise the horse with ground driving. This will require some coordination. For this reason, it will be better if you read some relevant books or at least watch a couple of DVD's related to ground driving a horse.
Now you need to train the horse to stay calm while you mount it. This will definitely take more than a few sessions because almost all horses get scared in one way or the other. Stay as low as possible while mounting the horse.
After you have familiarised your horse with your weight on its back, it is time to go for a ride. Be confident in your mind and keep your body relaxed. Choose a straight track to begin with and eventually move on to turning tracks. The horse may go fast in which case you should not restrain it. Instead, try to keep your weight balanced. This step may take a few months to perfect.