Joint the second side, and true on the table saw.

 Now that one side had been jointed, the second side goes relatively quickly. The main trick is to ensure that the first jointed side is held against the fence, so the second side is 90 degrees to the first....
Note the two unfinished sides are not straight or square...

 Note the 90 degree angle between the two jointed sides....

Most of the wood has been run through the table saw, and are ready to be rabbited (for the plywood shelf) and cut to length. Subsequent step would be the cutting of the mortises and tenons. The wood shown is now straight and square, measuring exactly 1.5 inches on a side.

Note however, I miscalculated on how much wood was required, and will have to joint and saw some more...