Plywood can be very flexible (depending on thickness) so proper support of the work piece is important when working with it. This “How To” assumes you will be cutting from a full 4 foot by 8 foot piece of plywood (or other sheeted timber material).
Find a flat space on the ground large enough to accommodate a 4×8 sheet with enough room to move around it.
Lay a 4×8 sheet of 1″ thick rigid insulation on the ground.
Set the depth of cut on your circular saw deep enough to cut completely through the plywood, but not so deep as to cut all the way through the insulation sheet. You’ll only need to cut about 1/4″ into the insulation. It’s important to double check this depth of cut because you would want to accidentally cut into the ground! In other words, take the the thickness of the plywood plus 1/4″ and set your circular saw depth accordingly.
Next you’ll need to find a straight edge (or something to act as guide for your saw). A straight 2×4 will do nicely.
Now you’ll want to find a speed square or some other layout device (bigger generally means more accurate) that will allow you to set to edge of the 2×4 90 degrees to the edge of the sheet.
Next establish your cut line and offset the 2×4 the “blade distance” you measured earlier. Square up your straight edge and place something to weigh down the 2×4 so it doesn’t move.