Here is a simple step-by-step method of boning a deer.
There are two primary backstraps, and it helps greatly to have a second person stand on the belly side of the hanging carcass and simply hold it firmly. This keeps the animal from “spinning” and allows boning to go more quickly and safely.
Begin a backstrap cut with a fillet knife by inserting the blade close to the backbone, and severing the loin away from the ribcage. Make slow, well-defined vertical cuts along the entire length of the backstrap, from the ham to the base of the carcass neck. As deeper cuts are made, the base of the ribcage is exposed and the loins almost peel away as cuts are made along the backstrap length.
Make a deep horizontal cut at the top of the backstrap near the ham, and a second horizontal cut at the bottom of the backstrap near the neck. Work slowly and carefully with the fillet knife, and the ’strap is easily removed in one large piece. Repeat on the opposite side of the backbone to remove the second. Trim fat and sinew carefully later.