The first three steps to position a saddle on a horse are these three steps:
Slide the saddle forward, then try raising the saddle higher, and then try lowering the saddle.
Dy doing this you assure you do not missfit the saddle, if you feel the saddle stop in the same spot repeatedly, well behind the shoulder blade, that is the perfect spot for the saddle. Resist the temptation to place the saddle too far forward on the withers.
All of these steps need to be performed with your horse standing squarely on level ground with his head and neck straight ahead, and with the saddle in direct contact with the horse’s back.