The last woodworking step is to trim the front of the cabinet. Because these units will not have any doors, we'll be simply trimming it with 1x2s.
To begin, verify that the cupboard assembly is exactly 48" wide. Then, cut one 1x2 to that exact length. Using finish nails, attach it to the bottom shelf and the two shelf standards, with the top of the bottom shelf flush with top of the 1x2 and the edges of the 1x2 flush with the outsides of the two standards.
Next, measure from the top of the bottom shelf up to the top of the carcase. Cut two 1x2s to this length. One is to be mounted with finish nails to each shelf standard, flush with the outside of the standard.
Now that the two sides have been trimmed, measure and cut a trim piece for the top and center shelves, and mount them each flush with the top of the corresponding shelves.