Glue the 1x3 laying down flat, so the 3 inch side has the glue on it. Make sure it is straight though before you start. Just glue the 1x3 on the edge of a sheet of plywood preferrably 1/4th inch to keep the most blade available to cut your finish piece.

 

The bottom of your circular saw has a flat piece of metal with the blade coming through. I call that the guard but I guess it could be the depth setting and angle face plate depending on how you look at it. When your glue has dried you just push the saw up against the side of the 1x3 with the guard or face plate flat on the plywood. When you cut all the way across, length depends on how big you made it, it will have the 1x3 on top of the plywood with about 2 to 3 inches of plywood sticking out the one side. The blade will be the exact width away from the 1x3 as your edge of the plywood since you just cut it with that width. Basically this makes your jig. The jig can then be clamped at any angle to any piece of plywood you need to cut out. Just clamp the piece down with the clamp on top of the 1x3 and then run your saw the length of the piece giving you a nice straight cut.

 

I hope that helps clarify a little. I am new at the forums but how do I go about posting a picture when I get the chance?