Where the front elbow rests on the ground depends on three things; whether or not a sling's used, the shooter's body shape and size and the stock's shape and size. Wherever the elbow is on the ground that enables the smallest holding area is right. For some folks it will be under the stock's fore end. For others, it'll be a little to one side; the lower arm angle may be anywhere between 0 and 20 degrees.
There are no hard and fast rules for the prone position except one; only your bones support the rifle; no tensed muscles at all 'cause they are constantly wiggled by blood pressure and are minimized when not strained. Whatever works is best. Without actually seeing someone in prone, I cannot suggest anything to help. But there are a few things to remember.
Changing the position from shot to shot of the front elbow, butt placement in shoulder, hand position under the fore end, cheek weld on the stock, trigger hand's grip on the pistol grip, will change point of bullet impact for a given sight setting.
Best advice without seeing someone is for them to get into prone then aim at something with a scope sighted rifle and relax; totally and completely relax. Take three deep breaths, let out half of the last one and totally relax. No muscle support of the rifle. Note the size of the area the scope reticule moves around in. After 15 to 20 seconds, start over breathing again. Move all parts of your body a little bit then repeat this exercise. Some position details will enable a smaller holding area. Keep at it until you can hold inside a 1 MOA area. Then get out of it and go back into it and start over. Learn to go back into the same position each time.
Buy Nancy Tompkins' latest book "Prone and Long Range Rifle Shooting" then read, learn, do, practice then perfect what she says. She and her husband along with their two daughters have probably won more matches and set more records shooting prone than any other group of four people on this planet.