Ryan Cleckner is the author of the Long Range Shooting Handbook and a former Ranger sniper. His book is an excellent primer on getting started in long range shooting.
In this video for the National Shooting Sports Foundation, Cleckner discusses the proper positioning of your head and scope for prone long range shooting. He makes an excellent point when he says if it isn’t right, change it. The scope and stock should adjust to you and not the other way around.