A hole saw is by far the easiest and most effective tool in helping you to cut a round hole in plywood -- far easier than attempting to cut a perfectly round hole with a jigsaw or hand saw. You attach a hole saw to an electric drill, and use the drill's circular motion to make a cut. Hole saws are readily available at hardware stores and come in a variety of sizes, from about 1 inch to nearly 9 inches in diameter.
Support the plywood on stands, in a vice or with clamps, so that you can carry out your cut safely and neatly.
Mark the centre of the hole you plan to cut with a pencil.
Position the point of the hole saw's centre on the pencil mark.
Start your drill, and apply light pressure to the back of the drill to begin making the cut. If possible, use a side handle on your drill for better control and precision.
Let up on the drill's trigger as you get nearer to breaking through the plywood, to reduce fracturing the wood. After breaking through the wood, gently press on the trigger and pull upwards to remove your hole saw from the plywood.
Your drill must be 18 Volts or larger to properly operate the hole saw. Use clamps on smaller pieces of wood to keep them from spinning with the hole saw.
Always drill straight to prevent binding. Wear eye protection while operating a hole saw. Never wear loose fitting clothes that could get caught in the hole saw.