Mark width using the actual piece
Mark the depth
Use a marking gauge or very carefully using any other measuring device.
Make a "knife wall" with a chisel
Using a wide bench chisel, place the bevel side towards the waste and hit lightly on a perpendicular plane to your workpiece. This makes a definitive guide for your saw and cuts the surface wood across the grain to avoid splintering.
Saw down the knife wall lines
Using a cross cut saw, cut down the shoulder lines being careful not to go too deep.
Work your way across the waste
Using the same perpendicular strategy, work your way across the entire waste portion of the dado.
Pare out the waste
Using a stabbing motion with the bevel up, pare out the waste in the joint.
Repeat steps 5 and 6 until you are close to your depth
Diagonal chops with your bench chisel can help to relieve some of the pressure in the waste but be careful not to fracture the shoulder lines of your dado with the shaft of your chisel.
Use a router for final cleanup
If you have a router plane, now's the time to use it. Otherwise, the poor man's router will do. If you're not familiar, check out our guide on how to create the poor man's router.