Release the tension on the thumbscrew that locks the sliding bevel by turning it counter-clockwise just enough so that the pressure releases and the bevel pieces move freely.
Determine the angle desired using a protractor, or align the bevel with an existing angle on another piece of wood by laying the bevel down on the material and moving the angle to match.
Turn the thumbscrew clockwise until the bevel locks tight again. Verify that you haven't changed the angle while tightening the thumbscrew.
Position the sliding bevel on the wood and align the lead edge carefully with the outer edge of board. Press down with one hand on the lead edge and slowly draw a pencil line along the angled edge to replicate the perfect angle on the wood.
Release the thumbscrew again and return the setting on the bevel to the original position for storage.