Slide on a pair of work gloves and safety glasses to protect your skin and eyes.
Measure the length of the dado you wish to create. Using a pencil make two parallel lines the width of the edge you'll insert into the dado.
Place a router -- powered off -- atop the wood. Adjust the router so the bit is at center between the trace lines. Place a 1/4-inch-thick guide strip atop the wood. Adjust the strip so it is parallel with the top pencil line but far enough removed to touch only the outer edge of the front of the router. Clamp the guide strip and the wood you are cutting to a work surface to prevent movement while routing.
Attach a dado jig to the hand router. Adjust the depth of the jig to suit your purposes. Average depths of dados range from 1/8 inch to 1/4 inch.
Place the router back on top of the wood. Adjust the router so the prefabricated arrow on the outer rim is facing the guide strip. Power the router on. Move the router slowly from right to left to cut the dado.