...with the rest of it sticking up above your shoulder line.
This is pretty important, as the butt can bite if you have it too low.
I was taught to angle my body so the rifle crossed in front of my face to get the off (forearm support) elbow under the rifle. Some high-power target shooters are advising to get the body and rifle in straight alignment with each other. I find the taking the recoil straight into the bulk of my body weight is uncomfortable and I prefer the angled position.
Seating the butt of the rifle into the shoulder pocket is critical. Try to get the same shoulder seating in each of the positions (offhand, high kneel, low kneel, sitting, prone,) as much as possible. There will be minor differences, as your head/sights position will change slightly as you get farther down on the rifle. The key is to get the same sight/eye alignment in each position. When shooting scope or peep sighted rifles this is not as critical as when shooting open sights.
Having a good instructor handle you to shape your positions during your training is good, but remember that it is YOUR body doing the shooting and the instructor's position might not fit you exactly.