Measure and mark three equidistant points along the length of the inner edge of the stud. These marks indicate where each 2-by-4 spacer will go.
Have a partner hold the first spacer in place over the topmost mark. If you do not have a partner, secure the 2-by-4 in place with a thin layer of wood glue applied according to the manufacturer's instructions. Proceed to the next step once the glue dries.
Toenail the spacer in place. Drill a pilot hole on the bottom face of the spacer at a point roughly 1 inch in from the spacer's outer edge and 1 inch in from the stud, angled toward the stud at 45 degrees. Drill a 3-inch screw in the hole at the same angle with your drill and bit. Install a second screw into the other side of the spacer, through the opposite stud following the same method.
Repeat Steps 2 and 3 to install the lower two spacers on the marks made in Step 1.