Build the top and bottom frames out of the 1x2 cedar strips. You'll be butting the ends together, so no mitering will be necessary. Fasten two 26" strips to two 13" strips to form each rectangular frame. (You'll need to ensure that the length remains 26"; to do this, butt the ends of the shorter strip against the longer strips. The thickness of the two longer strips will add an inch to each end of the shorter strips, increasing their length to 16". Do this at each end of the longer strips to form a rectangular frame 26" long and 16" wide.) Apply a bead of wood glue to the junctions of the strips; then nail together with a single nail in each junction in preparation for inserting screws to hold them more securely. Predrill the ends prior to screwing them together; this helps keep the ends from splitting. Then insert a screw in each corner.
Stand the two frames on their sides and apply a bead of wood glue to the inside face of the bottom side frames (the long sides). Attach four side panels to each long side, smooth sides out, and nail from the inside to hold them in place. Make sure the frames are flush with the panel ends on at least one side, or the bottom panels won't fit properly. If the panels are jagged on the other side, you can always smooth them with a trim saw later.
Because you're nailing from the inside, you may have to drive the nails at a slight angle. The advantage of this is that it conceals the nailheads from the exterior face of the window box. Repeat on the other side of the box.
Stand the half-completed box on its end and apply a bead of wood glue to the inside face of the bottom end frames. Attach two panels to each end in the same manner that you used to attach the side panels, once again ensuring that the ends are flush on the bottom. Repeat the process on the opposite end.
Check and adjust your box for square if necessary. Attach three bottom panels to form the base of the planter box, using wood glue and screws. The base will reinforce and brace the box. Drill several holes in the panels so that water can escape, or simply leave a gap between the panels.
Lightly sand any rough edges and corners to smooth out splinters. If you wish, you can stain, seal or prime and paint your planter to suit your home's decor. Because you used cedar, however, it can stay unfinished: cedar is one of the best lumbers you can use for exterior applications. This planter is a good size for potted plants, but you may wish to add a plastic liner and fill it with soil (add a layer of gravel first). If you do, don't forget to cut holes in the liner to line up with the drainage holes in the bottom of the planter.