Take A curved stick fallen from a tree and cut notches in both ends. Take a tight length of narrow (not thick) nylon clothesline and place a knot in each end. Insert the ends of the line into the notches on the wood so the line is rather tight. Rosin the string with fiddle rosin.
This will make a half-way decent bow for playing your saw. And it won't sever the string easily like is often the case with a regular fiddle bow.
Always draw the bow over the straight edge of the saw. If you do that on the side with teeth, the resulting note will be sharp.