Draw your bow back as if you were standing on the ground.  To shoot down from an elevated position bend at the waist paying attention to your anchor point.  It should be the same as when you shoot on level ground.  In other words your upper body should remain identical on every shot regardless of the shot angle.  To make elevation corrections bend at the waist.  Some people draw their bow back and then drop their arm to get on a target that is on the ground.  This causes the anchor point to slightly change in most cases causing them to shoot high.  Just remember bend at the waist to shoot down.  you can stand up at your computer now and practice with the ol' air bow.

From a trajectory standpoint, make sure you're using the correct pin for the horizontal distance to the target. Imagine a right triangle - the distance from your stand to the ground is the vertical leg, and the distance from the base of your tree to the target is the horizontal leg. The distance from your stand to the target is the hypotenuse, and will always be further than the horizontal distance. For example, if you're 10' up, and you range the target at 30 yards from your stand, the horizontal distance to the target is actually about 29 yards. Doesn't make much of a difference for a really fast bow, but if you're shooting under say 280fps it very well may mean the difference between a dead deer and a wounded one. The higher you go in the tree, the shorter the horizontal distance to the target is.

That's how pendulum sights work - you sight in the one pin at a horizontal distance, then when you're in the stand shooting at an angle, the pin swings out so that it appears to \rise\ in comparison to your sight housing, which makes you effectively aim the bow lower than you would at a horizontal target. The same goes for any projectile, especially with guns. The difference between ranged distance and horizontal distance is very apparent at the long distances many guns are capable of shooting.

Hopefully that makes sense. If it does, then you also have to understand that you must pick a different point of aim than if you were shooting at a horizontal target. If your aim is true and the arrow flies like you want it to, it will hit exactly where you aim, but if you aim at the same spot you do when you're on the ground, the arrow can likely miss all the vitals, as it is travelling at a downward angle instead of straight through. When you spot your target (the deer) and figure the distance, aim the pin at the vitals in the middle of the deer. Your arrow will appear to hit high, but this is because it is travelling at a downward angle. At 10' you don't want it to hit too high, but the higher you go in the tree, the higher your arrow needs to hit the deer.

Does that make sense?