First I mark both ends of the cut with a pencil.
Then I find something long and straight (like a board) and draw a line the whole way.
Then I prop the plywood up on something so the saw won't cut whatever is underneath.
Then I lay a board beside the cutting line, and clamp both ends with c-clamps.
Then I hold the saw blade right at the cutting line, find that my clamped board is not in the right place, so I loosen the clamps and move it over. (Repeat until it's right at both ends.)
Then I finally get to cut my board!
The only thing slower than my way of doing it: going back to the store for a new piece of plywood because I mis-cut something