This is an example that shows what I mean:
Before putting weight the saddle is positioned correctly
After putting weight, you will notice that the entire saddle has been pulled to the left
The most comfortable method of mounting, which applies the least pressure, is by placing the left hand on the lower neck or wither, and the right on the midsection of the seat or the far side of the cantle. Inserting the ball of the left foot into the stirrup. As you mount, you use the left hand to balance, and the left hand to pull along with hopping to gain momentum before pushing with the left leg. This ensures that the pressure is distributed evenly, and is easier on the horse, the rider and even the tack.