Once you've determined the height and depth of the cabinet, cut two identical pieces from one of the sheets of 3/4" plywood (in this case, you'll want each piece to be 18-inches x 24-inches).
Next, you'll need to decide whether the sanded side of the plywood should be on the inside or outside of the cabinet. On kitchen units, you'll likely want the sanded side out, whereas in the case of more utilitarian models, particularly when they will be installed one against another, you may want the sanded side in.
Set up a stacked dado set in your table saw to the width of the plywood (23/32"), and set the cutting depth to 1/4". You'll be cutting two 18-inch dadoes and one rabbet in each side (perpendicular to the 24-inch edge). You may use a router with a straight-bit in lieu of a table saw.
Set the fence on your table saw to 1/1-2" from the opposite side of the dado set. (In other words, if the fence is on the right side of your table saw's blade, set the fence at 1-1/2" from the left of the dado blade.) Start the saw and carefully cut the dado in the desired side of the stock with the 18-inch edge against the fence. Repeat with the other shelf standard.
Next, set the fence to 12-inches (in the same manner as before), and make another dado in the same side of each shelf standard, with the same edge against the fence.
Finally, set the fence to 24-inches (again, from the opposite side of the dado set), and make one more cut in each standard for the top shelf.