Where to place the saddle, per a guy who builds custom saddle trees: False "saddle fit rules" regarding the shoulder blades Saddle fit - Western compared to English Part 2 Saddle fit - Western compared to English Part 3
How does it change from English?
It changes in two ways - position and use of reins.
The position changes because the saddle tree extends much further back and cover a much larger area than the tree of an English saddle:
The purpose of a tree is to distribute the rider's weight, so if you want an even distribution of weight, you will have more weight to the rear of a western saddle. I'm in the process of switching from using an Australian saddle as my primary to a western one, and trying to put weight 'on my pockets' feels weird...so much so that I don't normally do it right now.
In the 'Old West' style, you had weight on your pockets and moved your hips with the horse:
Or in a more modern screen capture:
The waist absorbs the motion of the horse's back, not the small of your back.
Also, the western saddles I've got prevent contact between the lower leg and horse unless you move your leg in deliberately to do so. With an English or Australian saddle, I always have some contact with my lower leg. The western saddle, however, forces my lower leg away from the horse, so it takes a deliberate effort to touch the horse below my knee.
Use of Reins: The norm is to ride with one hand and slack in the reins. You do not ride with contact. Instead, the weight of the slacked reins amplified by the leverage of the bit forms a type of contact. The horse will have no trouble feeling, but it isn't a direct feel of your hands. It is more a feel of your hand's position.
It is also OK for general riding to use snaffles, and even contact if the horse knows how to be ridden with contact. But the true western approach is to neck rein and to ride with the horse's head moving freely.
From the horse's perspective, the big thing is to make sure the horse has been trained to do what you want. If a horse has not been trained to be ridden with contact (our gelding Trooper), then it has to be taught (if you want it). If a horse is used to lower leg contact, that is fine. But Trooper was a ranch horse who had never been ridden that way, and he got pretty excited when I first tried riding him in an English saddle with my legs wrapped around him!
Note: Bridles and throatlatches. If you go with a one-eared bridle, I recommend you only do so with a leveraged bit. The leveraged bit tightens the bridle on the horse's head when the reins are pulled. With a snaffle, pulling on both reins loosens the bridle...and yes, I had a one-ear bridle come off when ridden with a snaffle.
This is Mia at the end of a ride mixing an English saddle with a western approach to reins.
Mia in her new, used western saddle:
Good luck, and enjoy learning a new style!