Here is a sketch of the basic design. For those requesting a drawing with more detail here is a PDF version . With the exception of the base, which I made from half inch ply, the rest of the materials are 3/4" stock. I used leftover maple flooring scrap for the guides since, when ripped to a 45, trued up with a hand plane and waxed, is less prone to wear.

The size is primarily dependant on your job. I made mine to fit 18" panels because that was the size of box I was making. I would imagine one could make it as large as they wanted so long as they beefed up the top guide to avoid deflection. I also added hold-downs using "T" nuts and smoothed over carriage bolts.

To use, I rip the panels just oversize and clamp the piece in the jig with the workpiece squared to the far fence and the cut standing just proud of the guides. Then, holding a block plane at a skew with the toe riding on the base, make light passes until the face is true. Then I swing the panel around, align the trued up edge to a pencil mark on the base and true up the other side.

It is important to keep in mind that both edges of the board should be parallel and when you construct the jig, the guides must be perfectly square to the fences (or at least the one you will be aligning to). I also found that, even though I provided for clamping the jig to my workbench, I rarely do so. The key is light passes and a sharp plane.