Circular Saw Plunge Cut
If you need to cut a large hole, and if you want the hole's sides to be nice and straight, a circular saw works great. This cut is made with a technique called plunge cutting. The idea is simple, and the technique is easier than you might imagine. But before you take on this technique, make sure that you're comfortable using a circular saw.
Start by adjusting the saw for the correct depth of cut. We suggest setting the blade to cut no more than 1/4" to 1/2" deeper than the thickness of the plywood you're cutting. With the depth set, line up the saw over the cut line on one side of the hole. Rest the front of the base plate on the plywood, and hold the blade above the workpiece. Use your front hand to hold the front edge of the plate firmly against the plywood.
You'll have to hold the blade guard open, so make sure the blade isn't touching the plywood. Then start the saw, keeping a firm grip on it, and slowly pivot the blade down into the workpiece. Once the shoe sits flat, complete the cut. Then repeat the process for the other sides. You won't be able to cut all the way through at the corners, so use a hand saw to finish up each cut.