Step 1: Cut a sheet of fabric that is the size of the mattress. Then cut 2 side panels and one end panel the height you want, and each panel 2-3 times the length of the mattress, depending on how much fullness you would like.

Step 2: Hem 3 sides of each panel, as shown below.

Step 3: Gather the top edges to fit the mattress.

Step 4: Sew the panels to the sheet of fabric that fits the mattress, overlapping the corners 6" (to accommodate corner posts). See sketch below. That's it, you are finished!