never done it myself but there's a guy who busks with one outside the Roman Baths at Bath...

as best as I can figure it

Sit down and place the handle of the saw between your knees with the teeth pointing towards you (thick sweater of the Fair Isle class recommended)

Grab the tip of the blade furthest away from you with your left hand and bend it slightly to the right. at the same time press downward so the blade flexes in a curve to the left.

Take the violin bow in your right hand and draw it gently across the smooth edge of the blade (furthest from you). you should get a note

By adjusting the amount of bend and flex you change the tension on the blade and so alter the note