here is my favor t way of doing that John Doney or any one else
I like to use this method for cutting dadoes is to use a straight cutting bit on a router. When using a router to cut a dado, keep in mind that you'll want to dial down the bit speed quite a bit and adjust the depth for more than one pass to keep from burning the bit or the wood.
Use a straight edge to guide the router, to ensure a straight path. Be advised that using a 3/4" router bit will cut a dado slightly larger than a 3/4" sheet of MDF or plywood (which is really 23/32" thick). While 23/32" bits are commercially available, using a 1/2" bit and two passes would provide the desired results. I for got to add I like to use 1/2" shank and up in size this makes the bit very stable
Points to Remember:
When cutting a dado, try to avoid cutting any deeper than 1/3 of the way through the stock receiving the dado, to keep from weakening the stock. For instance, when cutting a dado in a 3/4" shelf standard, make your dado cut 1/4" into the standard.
Also, there may be times when a dado shouldn't be cut the entire length of the stock. In this event, it may be best to cut the dado on a router table.
Set the fence to the proper width and mark the start and stop points for cutting the dado on the fence with a pencil. Then, after starting the router, slide the stock over top of the head (keeping it against the fence) and ease it down onto the cutter. Slide the stock forward to the stop point, then back the stock up an inch or so before turning off the motor. Wait for the cutting head to stop spinning before lifting the stock off of the table.