This is by no means my idea as it has been around for quite a while.
However, as my table saw couldn't accommodate large widths, I needed to be able to make quick and accurate straight cuts and this was the easiest way of achieving it.
Using the skill saw (circular saw) and following a line is ok for rough cuts but where you need to be accurate you really need a fence for the saw to run along. Previously I used to have to clamp a piece of wood with a straight edge the exact distance away from where I wanted the cut (the distance from the edge of the saw base to the blade). With this guide you just line the guide up with the marks of where you want to cut and clamp it down.
I just used some faced hardboard I had lying around (I think they were the sliding doors from an old bed base). You could use some 1/4 inch ply or some other thin material. Ideally it doesn't want to be too thick otherwise you will be limited as to how thick the boards you can cut can be.
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