Measure the width of the mattress; twin beds typically run from 36 inches to 39 inches. Add 2 inches to that measurement, so that when the bed is made the headboard will still appear fitted. Mark the width on the plywood base. Then draw out a rectangle pattern using a yardstick. The headboard can be as tall as you would like but should not be shorter than about 2 feet. Note that the base will have to be set about even with the mattress but still be seen when a pillow is placed on the bed.
Cut the plywood according to your pattern, and sand the edges smooth. Cut the same pattern out of the foam using a craft knife. Spray one side of the plywood pattern with fusible adhesive and attach the cut foam piece evenly to it.
Roll out the fiberfill across the top of the foam so that there is enough to fold over behind the plywood; cut it to size. Roll out the base piece of material in the same fashion and cut. Choose a fabric that matches the room's color palette or that matches the sport base. For example, use green for soccer, brown for baseball or tan for basketball.
Lay out the fabric on the floor or on a table and place the fiberfill on top of it. Then center the plywood base on top of the fiberfill with the foam side down. Draw up all the materials at the midway point on one side of the board. Bring the material up taut, and staple it to the back of the board every 2 inches. The material should be snug but not bulging. Do this on each side to secure the material in place and repeat around all the edges. Fold the corners in an overlapping fashion to create a clean corner. Trim away any excess material.
Draw a pattern of the chosen sporting item on the backside of a different colored piece of material -- white for a baseball, orange for a basketball, etc. You may choose several or just one item for your headboard. Cut out your pattern and lay it out on the finished headboard to create a design you are happy with. Paint lines or other designs on the patterns to create your sports theme, such as black lines on a basketball. Apply the completed pieces to the headboard using the fusible adhesive.
Locate the studs on the wall behind the bed and mark them. Attach the bottom section of the flush mount to marked areas of the wall in as wide an area as you can within the range of the headboard. Place the headboard over the mounts at a tilt, then mark the headboard where the receiving mount should be placed. Secure the mounts to the headboard, then simply slide the mounts over the mounts attached to the wall.