1) Figure out your dimensions. Cut a piece of 3/4" plywood the height of the cabinet, and double the width---plus at least 1/4". Use a router (if you're lucky enough to have a table saw with a dado blade you probably know more about building cabinets than I do) to cut a track everywhere you want a shelf, and the bottom, and the top.
I just clamp my 4' level for the staight edge guide...Here's why you made the plywood piece twice the depth of your cabinet-----
Now you can cut it apart intot he left and right sides and the kerfs will match up perfectly. With a steady hand and a skilsaw you don't really have to have a table saw. Run both sides through so they are the same (that's why you added at least 1/4" , to make up for the saw blade kerf)
Whatever material you're using for the back, cut a rabbet along the back edge. I found some 3/16" luann for $6 a sheet so the rabbet is 3/16" deep.
Flip the carcass over and measure the height and the distance between the left and right rabbets. Cut your backing material accurate and square---this is when you square up the carcass, Nail it along one edge, then just force the carcass in line with a perpendicular edge. Mark where the shelves are and nail it there, too. If you have to cut out for plumbing or electrical, just bring that backing inside and mark and cut now, then put it on.