Turkeys typically come with the neck and several organs inside them, which are sometimes in a paper bag. Take out a large cutting board and poultry shears (you'll use those in the next step).
Lay the turkey with the back down and the breasts up. Remove the innards and set them aside (these can be thrown away or save them in the freezer to make broth with). If there is a plastic handle in the turkey, remove that as well.
Turn the turkey breast-side down so that the backbone is facing up. It is easiest to cut with the head of the turkey facing you, meaning the wings are close to you and the thighs are further away on the cutting board. Use your hands to identify where the backbone is.
Take the poultry shears and start cutting to the side of the backbone (the picture is shown cutting to the right of the backbone). You will be cutting through some bones so expect some resistance but you will be able to keep cutting.
Make an incision all the way down until you finish cutting through the first side (all the way to the bottom end of the turkey). Next do the same thing on the other side of the backbone, cutting all the way down and through the other end of the turkey. Lift the backbone out and set aside. You can freeze it in a plastic bag with the other discarded pieces to make turkey stock.
Using a sharp knife (or if you can separate your shears, use the sharp edge of one of the sides of the shears), make a shallow cut down the middle of the inside of the turkey. This will cut into the cartilage and help the bird lie flatter when cooking.
Push the two sides of the turkey apart a bit with your hands to help it flatten. Then flip it over so the inside of the turkey is laying flat on the cutting board. Put your hands on top of the bird and push down to flatten it more. You may hear some of the bones or cartilage break, which is fine.
That's it - you now have a spatchcocked turkey!