This story shows you how to create a dado joint.
Use a carpenter's square and pencil to mark the dado locations and width on the wood. If the project uses plywood shelves, double-check their thickness. Don't assume 3/4-inch plywood will always be 3/4 inch thick. Dimensions vary with the manufacturer, and if the dadoes are not the exact width of the shelves' thickness, you'll have a bad fit.
Cutting a dado with a portable circular saw requires setting the blade to the depth you want the dado, usually one-third the wood's thickness. (Be sure the blade is perpendicular to the saw's foot plate.) Next, with a straightedge as a guide, saw kerfs in the wood to shape the dado edges. Then saw narrowly spaced kerfs between the outside cuts.