Lay your piece of plywood on two sawhorses so you can work around the plywood from all sides.
Mark lines on the plywood from opposite corners. The two marks will meet in the center of the piece of plywood. This point becomes the center of the table.
Drill a 1/8-inch hole in the end of the straight edge at a distance of one half the diameter of your table.
Place a nail in the end of the straight edge and tap it into the center of the plywood. This will give the straight edge a pivot point to lay out the round top.
Place the pencil in the other hole and mark the plywood by rotating the straight edge around 360 degrees. This will outline the round table top.
Cut the plywood with the jig saw slightly larger than the outline.
Carefully take off the excess wood with a belt sander. Use a medium grit sandpaper belt to avoid taking off too much wood and causing the table top to end up out of round.