I didn't find a guide to making a circular saw guide that was made this way, and it was shared with me years ago. Before I bought a small table saw, and even after because it is often easier than dealing with sheetgoods on a small saw, this is how I cut plywood. This guide will be as precise as you can mark a board and clamp. To aid with precision, I use a mechanical pencil- I have never understood the use of a carpenter's pencil, but as a cabinetmaker it shouldn't make a lot of sense. Another option is to use a metal scribe.
But, let's get on to the saw guide.
Step 1: Base and Guide
First you need a board with a very straight edge on it. If you have the tools to do this yourself there is a chance this saw guide is not for you anyway, but it could be done with a jointer, or taken to a lumberyard and ask the to straightline the board for you. It shouldn't cost much at all, and the dollar or two is worth the investment to insure that the rest of your cuts from here on out using the guide are straight and true.
For example's sake, let's say the board that you straightlined is about 4 inches wide, and your saw's base is about 8 inches wide. The board you choose for your base needs to be the width of your guide board, plus the width of your circular saw footplate x 2. [Guide board width + (saw base width x 2), so in this example 4" + (8"x2)= 20" wide]. The length of these two boards depends on how wide a board you want to be able to cut.
Ok. Mount the guide board to the base, centering it as close as you can. this does not have to be uber-precise, but does require that on one side when you put your saw on the base up against the guide it will cut the entire edge off.
The cutting of the edge is very important- this is how this guide is so much more precise than other guides that require you to measure the offset and yada yada yada to get the guide to put the saw where it will hopefully cut..
The first picture shows the assembly uncut. With the saw that you plan to use the guide for snugly up against the guide, cut the excess wood off of the base keeping the saw along the guide. The second picture shows the saw part of the way through the cut.
Step 2: Using the Guide
Now that the base has been trimmed by the saw that will be used with the guide, the edge of the guide should exactly match where the edge of the saw blade will cut.
When you have a board you wish to cut, measure from an edge and make two marks on it to line up the guide. The nice thing about this is that since the saw cuts where the guide actually is, you don't have to do any further math. If you need to cut at 36" from an edge, make two marks 36" from that edge and clamp your guide exactly on those marks- this is where your saw will cut.
I didn't model in the clamps, but the reason that you need material on the other side of the base is so that you can clamp the guide down more easily.
You can see in the pictures where the guide should be attached in relation to your marks.
Step 3: Using a Router
This guide will work equally well with a router- the only problem is that you need to have an edge for each different diameter router bit for it to work precisely. You could use each side of the base for a different router bit, so if you have a couple that you typically use for straight runs you can size each side for one of those bits.
Otherwise you need more than one guide for multiple bits, but it will work just as well with a router.