Haha ok! Bare with me then :)
First and foremost rule is no hand print on the chest, and the color red is never used. Red=blood. A rider who was critically injured would send his horse home with a sort of "I'm not making it back" notice by getting enough blood on their right hand to leave an impression on the horse's chest. This way when the horse wandered home or what found without his rider, the tribe knew what happened.
A sun goes on the horse's right hip/flank area. It stands for happiness, peace, hope. It is also a prayer for the patching front of you and your horse to be figuratively "well lit".
A bird is drawn on the left flank/hip area. That is for balance.
A circle goes around each eye and around each nostril. These are for better vision and better sense of smell.
A little forward of the horse's croup should be an arrow. The arrow should zig zag before coming into a point facing your horse's head. This is for swiftness. I always drew my arrow backwards (point towards tail) and my grandpa would always tell me that's why the horses I rode ran backwards. Haven't tested that theory out yet, ha!
Arrows pointing up should be painted on all 4 legs. This is for speed. These arrows should be straight (no zig zag)
Arrow head on each hood for sure footedness
A left hand print on the horse's right shoulder meant the horse had been given to you by a special person in your life (such as a good friend, etc and that you and your horse's bond now honor that individiual).
Here is where I've noticed it really gets a little hokey for some people... You have to take a moment with your horse and really think of something that sums up your relationship. It can be a letter in your language, a word, numbers, a Christmas tree... It just has to be something that holds a special significance for you and your horse and you have to feel strongly about it. This is YOUR prayer. And it is specific to THAT paint session/following ride. You do not share the meaning of your prayer.
If you go out on a ride and it's bad. Your prayer didn't work. It means you didn't pick something that was special to both you and your horse . You are still thinking of it as an animal (which it is..don't get me wrong) instead of your means of livelihood, your working partner, your friend.
That is basically your everyday prayer paint. For this it is important to keep the energy low and calm. For a war horse the energy is different. More hyped up and violent. And ther are different symbols that stand for how many men you and your horse have killed, etc. hunting horse's have things like buffalo tracks and fences on their sides :)