Every horse person needs to understand saddle fitting, both for their comfort, but ultimately for the comfort of the horse. When riding, if western saddle fitting has not been properly done, it can affect not only the performance you get from your horse, but can ultimately cause injuries. Taking the time to carefully measure your horse and get an accurate size and width makes all the difference on the trail or in the ring.
The first step in purchasing a Western saddle is to measure the size of your horse. In order to do this, use a heavy coated electrical wire and some cardboard. Use this to make a "form" (think how a dentist makes a mold for your teeth). A qualified tack representative will be able to help you use this information to find a good saddle.
To make your mold, start at your horse's shoulder blade. Bend the wire across your horse two fingers behind the shoulder blade. Make sure the cord is flat, and then trace the shape onto your cardboard. You will use this as a guide for saddle fitting - not a definite choice. Think of tracing your foot for the size of a shoe - there is not a guarantee it will fit, but it definitely narrows the choices. Hold the cardboard under the saddle to see if it presses in or is just too big.
You need to try the saddle on your horse once you have a suitable choice. Do this while your horse is level, and make sure that you have the permanent blanket you will be using with the western saddle fitting. Slide the saddle into the natural stopping point (just behind the shoulder blade). Girth the saddle and make sure that it is level. Do this by eyeing the skirt of your saddle to make sure it isn't slanting.
Make sure it isn't pinching your horse by running your hands under both sides of the withers.
The horn needs to be a couple of fingers above the horse, and the horse should have room between his hips and the back of the saddle. This is more common in the shorter bodied breeds, such as Morgans or Arabians, so if you have this type of horse you may need a shorter than usual skirted saddle.
Ride in your saddle at all five gaits. Check your horse's reaction to the saddle, including his ears, whether he is bucking, or even biting towards his sides. These are signs that your hose is not comfortable.
The horse isn't the only one who has to be comfortable! You, the rider, will be spending countless hours in your seat. Make sure that you can place you hand between your leg and the pommel, and check the position of the stirrups. If you aren't comfortable, it doesn't matter how well it fit's the horse! Western saddle fitting if for both horse and rider.
Once you have dismounted, you will want to check your saddle again since it has settled onto the horse. If everything looks good, go ahead and take the Western saddle fitting as a success and happy trails and shows!