Wiggle one piece of it under the back of the turkey between the turkey and the rack on the roasting pan. Take the ends of the string and make a bowtie knot on top of the turkey breast tying the wings to the turkey's sides.
Take the second piece of string and do the same thing, only this time tying the legs in position a bit lower down on the turkey.
Finally, take the third piece of string a tie the ends of the turkey legs together, tucking the flap of extra skin and fat underneath the legs, thus holding it in the position seen below.
Some turkey roasters suggest tying, or sewing the entire body cavity shut. This seems unnecessary to me and even undesirable since it will impede the flow of juices and drippings into the pan.