Measure the width of the top of your bed, which is the portion of the bed that will rest against the wall, with a measuring tape.
Choose shutter doors that are equal to or larger than the width of the top of the bed. For example, shutter doors that are 2- to 8-inches wider than the top of the bed will work as a headboard. The exact size of the shutters will vary as there are a variety of types of shutters available.
Pull the bed slightly away from the wall. Lean the shutters behind the bed to determine exactly where you want to place them. For example, if the shutters are exactly equal to the size of the top of the bed, position them so that they sit evenly behind the bed. If the shutters are slightly larger -- 4 inches, for example -- position them so that they are centered behind the bed with an extra 2 inches, extending past each side of the top of the bed.
Drill one 4-inch wood screw through each of the upper and lower corners of the shutters and into the wall to secure them in place. You'll need four screws for each shutter.