Making sure your saddle fits properly on your horse is important for your horse’s comfort and if not fitted properly it could end up in harm for your horse.
When fitting a saddle never fit a saddle on with anything underneath. Make sure that your horse is on flat ground. Place the saddle slightly up your horse’s neck then slide it backwards, so that the saddle fits directly behind the shoulder blade. The shoulder blade is  underneath the wither area.
Look at the saddle from the front and behind. You should be able to see through the gullet or channel running through the saddle. You should be able to fit roughly 3-4 fingers when you place your hand at the pommel area, which is the arch at the front of the saddle.
When looking at the saddle from the side, the seat area should be horizontal. When viewed from the front, the saddle should fit comfortably around the horse’s shoulders.