Secure the piece of wood that is to be cut with the dovetail joint into the jig. This is done by placing the board in the jig with the end of the board that is to be cut facing downward against the teeth of the jig which are laying on a flat surface.
Tighten the clamp on the side of the jig that holds the board in place, making certain that the board is pressed firmly against the block at the end of the jig. This makes the cut reproducible if you are cutting a dovetail in both ends of the board.
Turn on the router.
Press the wood into the router blade between the dovetail jig's teeth. The router will cut a V-shape into the wood between the teeth, leaving behind the dovetails.