There are several vids on YouTube about shooting positions. Type in ballistic tips and they are field sports channel clips.
A few mistakes i see all the time are people trying to sit up too high. Get right down, lean forward and jam elbows into knees for bone support if youre not using sticks. If going off sticks, again, get right down but for a right hander get you right elbow onto your right knee, that stops pivoting round the sticks and makes you much steadier. Do what you like with the left, just whatever helps. Then, most important, make sure you're body is neutral. Everything in the middle of it's range of movement so there is no stress. Finally before you shoot, acquire the target. Shut your eyes and relax. Open eyes and if you're not still pointing at the target move round so you are. Takes ages to write but seconds to do if you practice. Also if you're going off sticks, don't for crying out loud shoot off the side of a pair of sticks, only from the V at the top. There is no strength in a stick in tension, only in compression. Try it. You will bounce all over!
P.s. If you have lower back problems like me, forget it and do it from kneeling!!