At this point, we need to determine how you will be hanging the cabinet. In kitchen installations, you'll likely use a piece of 3/4" plywood as the backer of the carcase, and screws will be driven through this back to secure it to the wall.
However, in our project, we'll be using a French Cleat wedge bracket that we'll fashion out of a 2x4 in a future step. As such, we don't need the additional weight of the 3/4" plywood on the back, so we'll use 1/4" plywood. In this step, we'll be cutting a rabbet to accommodate this back.
Set the stacked dado cutter to 3/8" depth and 1/4" width. Then, set the fence on the table saw to 18-inches from the opposite side of the dado set (as in the previous step). Determine which 24-inch edge of each shelf standard will serve as the back, and cut a rabbet (on the same side of the stock that you cut the dadoes into).
Next, you'll need to cut a similar rabbet into one of the two 18" shelves. This will be cut into the back, top-side of the bottom shelf (1/4" wide and 3/8" deep).
If you are modifying these plans and will be using 3/4" plywood for the back, you'll also want to make a rabbet into the back-bottom side of the top shelf of the cabinet. However, for the method we'll be using for hanging our unit, this rabbet will not be necessary.