A jig is a device that holds a piece of work and guides the tools operating on it.  It is easy and inexpensive to create a circular saw guide.  Instead of trying to explain it myself, I’ll just point you to this excellent video from This Old House.

I made my jig out of 3/4″ particle board.  It is a bit heavy but I can walk or kneel on the side of the fence opposite of the saw.   Shown below is the setup about to rip a board. I set the blade depth so that it cuts into the scrap foam pieces.  This guide if 6′ long but I made a shorter one for smaller pieces.

If you make a smaller one, you might want to make it dual purpose so that it has a guide for your router too.  Here is an image from WoodSmith magazine that shows the guide for both a circular saw and a router.