The side-pass should be very easy however many people have trouble with side passing their horse.
Starting out probably should be done before you even get on your horse using groundwork schooling because your horse may have no knowledge of moving away from pressure. If this is your situation, start with grooming your horse using a simple brush and while holding your horse’s head short, gently and very patiently bump on the horse’s side where the back cinch would be located.
Just pushing on a horse’s side or squeezing with your leg won’t get some horses to move. This teaches your horse to move away from pressure without the distraction of side passing from the saddle over an object or between two poles.
I personally like to teach a horse to move off my leg by waving my leg rhythmically against the horse’s side. Eventually there is really no contact, and this turns into more of a signal in time.
The best place for your hand position is halfway up the horse’s neck just in case he tries to back up. You will find that you won’t need to move your hand and it frees up the horse’s shoulders so that he can cross his front feet making the it easier for him to execute.
The only time I will touch a horses mouth is to keep them straight or to keep them from walking forward. The moment my horse is side passing I am keeping my reins against his neck. It is also important to keep the reins loose or soft.
Your seat is very important and if you are not on the correct side of the horse (meaning your weight) this can confuse the horse. When executing the maneuver left you want to lean to the right side of the horse. As you are waving your right leg to go left, it helps to look at the leg you are waving with rather than the direction you are going in. If you tip their nose towards the side pass it will make it harder; therefore, in the beginning it is better for the horse if you let him slightly tip his head away from the direction you are moving in. This allows him to cross over easier.
The first week or two depending on the horse I will ask for three steps only each way. Each week I will increase the amount of steps until I can ask for ten to twenty steps side passing easily.