#1 Body straight, gun straight parallel to the spine.
A straight spine is critical in my opinion, yes you can shoot very well laying at an angle. However once you start getting into calibers with recoil laying at an angle will cause your body to "twist" during recoil moving your muzzle up and to the side. This makes it almost impossible to keep a natural point of aim shot to shot, and if you miss its impossible to accurately measure how far you were off. All you know is you were "about a foot and a half high and maybe am foot left" Laying straight puts your entire body behind the rifle, and under recoil you get pushed straight back then you come straight forward again. Even with punishing rounds like a .338 if you lay straight behind the gun and load the bipod you can maintain visual with the target during recoil and you will come right back on target very close to your original natural point of aim, allowing you to accurately measure the difference between point of aim and impact. That way if you did miss on a game animal you can make accurate corrections on your own, and you dont have to rely on a spotter to guess.
#2 My feet are a few inches on either side wider than shoulder width apart.
Keep your feet as flat as possible, if you cant lay them flat go as far as you can, if you try to balance your feet with your ankles in the air any movement will go straight to the muzzle
#3 My Elbows are pushed out in front of me, I put my left hand on the rear bag and my right on the grip. I slightly push back with my elbows to coil up a little bit then put the rifle in the pocket of my shoulder then relax my muscles and "lean into" the rifle loading the bipod legs, Also I just rest my face on the stock, I dont put any pressure or try to lift any off, if you just use the weight of your head to guage pressure you are alot more consistent.