Measure the distance from the blade of the band saw to the upright part of the saw. This distance is the maximum width of the cut you can make with the saw, as the upright component is solid.
Use a tape measure to choose the position of your cut on the sheet of plywood and mark the location with a pencil. Hold a straightedge against the pencil mark and draw a line along the length of wood.
Set the piece of plywood on the table of the saw and, if the plywood is large enough, ask a family member to hold the other end of the wood steady. Line up the wood so the pencil line is directly in front of the blade.
Turn on the saw and slowly push the wood through the band saw blade, keeping the wood straight so the cut follows the line. Have your assistant steady the wood as it moves across the saw. Ensure he doesn't pull it, as doing so could cause the blade to bind.