Place large sheets of plywood on 2x4s laid across sawhorses and positioned to support both sides of the cut. Clamp a strip of plywood or other straight material to the work piece at the right distance to guide the saw shoe while cutting the line. Walk alongside the plywood as you cut, holding the cord to make sure it doesn't get snagged.
Tip: For narrow rips, keep the saw straight by holding the shoe with your free hand and bracing your forefinger on the wood's edge. You can also use a pair of locking pliers (below) or a rip-fence accessory.
*sawing with the grain