Measure the length needed for the dowel rod at the top of the 1' x 3' panels. The 1' x 4' panels will probably be warped, so if you measure at the location that you want to attach the dowel rod, the size will be wrong.
CUT AND ATTACH
Cut the dowel rod to the measured length. Drill pilot holes in the ends of the dowel rod. Attach the dowel rod to the panels as shown in the picture.
If you look closely at the picture at the beginning of this instructable you can see an alternate method for attaching the dowel rod. For the first dress up clothes caddie I made I cut an oversized dowel rod so that it passed through the panels and then attached it using some 1" x 2" blocks. The choice is up to you.