The back is the simplest component in the frame, as no additional cuts are necessary. Assemble the components as shown below, match the colors to the colors shown on the materials list. All joints should be both glued and screwed using 1.25" length deck screws. The corner joints should each have four screws, two from each side in a diagonal pattern. Try to avoid putting screws now into the area you will need to eventually mount the hinges. The exact position for that will be determined by the hinges you purchase.
Required materials:
1x3 nominal stock
(2) @ 14" (red)
(2) @ 9" (yellow)
(2) @ 32.75" (green)