Good advice here so far. Bend at the waist. I also found it helpful to bring the bow from the ground up. Aim for a spot that will give you a perfect exit hole if the arrow continues through the deer in a straight line. I have found that if I simply aim at the deer like I do on the ground by picking a double lung spot, I tend to hit high when I am shooting from a treestand. Especially if the deer starts to "string jump". Aim for an exit hole into the off "arm pit" area. This should give you bioler room hit if perfect. Lung and liver if back a bit, heart if low, lung if high and both shoulders with arteries if forward.
Also remember that anything quartering to you brings the shoulder blade / scapula into play as you are now in a tree stand and squared up to this bone. Do not try and shoot through this bone or around it. I have seen and experienced to many failures when this bone comes into play.
Good luck, stay safe and post some pictures and stories.