Are you struggling with the girth or tensioner to tighten the saddle properly and prevent it from slipping? No way! Start by doing one thing at a time, like positioning the saddle correctly. Tight yes, tight no.
Horses that wear a too-tight belt tense up and don't like to be saddled. Seriously, it's just like putting on a girdle that's three sizes smaller than you need it to be.
How comfortable do you think you are in this machine? If you overtighten it, chances are the horse will reach out to bite you, huff, or kneel. Therefore, a well-fitting girth is the ideal solution, at least in the horse's mind. Remember to position the saddle correctly and you shouldn't have any strap issues.
The saddle should sit firmly and be level on the horse's back. That said, there is so much variation in the back, withers, and croup of horses, as well as the saddles themselves, that it is sometimes a challenge.
Anyway, assuming you can get the saddle level on the horse's back, it should now be on/in the rider's center of balance - just behind the horse's natural balance point at the stop. As the horse moves, engaging the hindquarters, its natural balance point (just behind and slightly above the elbow) moves back and under the rider as the horse rounds its back upwards. Now the horse, saddle, and rider should be in balance.
Place your saddle pad or blanket over the withers and well forward. Then place the saddle over the withers and forward. Slide the saddle and saddle pad back until they snap into position, behind the withers, and level with the back. This can make the saddle position feel too far back. This is not the case.
The saddle groove will be slightly above the withers. The saddle is level and the back of the saddle does not press on the lumbar region. The strap will not be immediately behind the elbow, but several inches behind the elbow. Hmmm, sounds strange if you were taught otherwise, doesn't it?
If the saddle is too far forward, well above the withers, and the girth is directly behind the elbow, the movement of the horse's shoulders is restricted. Stride and lateral action are affected. Correctly positioned, the saddle will not need to be tightened, as it will stay in position naturally with only a tight tightening.
Once the saddle is in place, check the pad or saddle pad and lift it up the gullet so that at least two fingers slip easily between the withers and the saddle. If you have correctly positioned the saddle, your horse will not have to suffer from the withers or the kidney.
You can spot problems from the ground by stepping back and observing the saddled horse.