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This process is fairly simple, and you'll be able to use the jig every time you want to cut a piece of wood.  Use a sheet of 1/4" plywood as your base. Cut a piece about 10" wide and about 4' long.(If your project is much larger, make the base longer to provide more stability.) Cut a piece of 3/4" plywood to form a "fence." The fence should be a few inches wider than the width of your saw's motor and should be as long as your base. Align the base and the fence at the ends and one long edge. Use wood glue and screws to glue the base and the fence together. Clamp the jig to the edge of a work table. If you don't have one, you can secure a large sheet of plywood to two sawhorses and clamp the jig to the edge of that. Align your circular saw with the fence and cut along the base. This will trim any excess and make sure the jig fits your particular saw perfectly. Mark the line you want to cut on the back side of the piece of wood: this will keep the front of the wood from splintering. Align the cutting jig with the line you just marked. Clamp the jig to the wood. Place the "good side" of the wood face-down. Circular saws cut clockwise, which means that most of the splintering happens on the side of the wood facing the blade. Pushing the saw away from you, move the saw steadily and slowly along the jig. You should have a straight cut!
Create a cutting jig for your saw. Use a pencil and a straightedge to mark your line on the wood. Place the wood and cutting jig on sawhorses or a work table. Align your saw with the fence on your jig.