I made a jig for Dados, but you can get away with clamping a straight edge over both of the cabinet sides.
Lay both sides of the cabinet on the flat and against each other, align them to ensure both are even then clamp them to your work table.
Decide what thickness the shelves will be, (1/2", 5/8" or 3/4") and mark where you want the shelves to be placed on one of the cabinet sides and use a straight edge to carry the mark over both sections. I mark the top or the bottom of the shelf not both.
Know this it is likely that your plywood isn't exactly 3/4" or 1/2", that it's undersized and you will need a cabinet grade router bit for the job. Standard 3/4" cabinet grade, AC and MDO plys are actually 23/32" check to ensure exactly what it is.
Take a test run against the straight edge on some junk to gauge the offset required to hit your marks on the cabinet sides then set accordingly on the sections.
Ensure that the cabinet sides once set for marking do not move prior to or during the dado process, check for alignment between each dado for safety
Never bite the hand that looks dirty!
The more you know the more you're worth