Cut the pieces for the legs. Cut a 30 degree angle in each end. Mark the taper, then cut using a jigsaw, a bandsaw, or a tapering jig on the table saw.
Cut the pieces for the stretchers. Cut a 45 degree angle in each end of the long stretchers and the short stretchers. Also drill pocket holes along one long edge of each piece (to attach to the bottom of the cabinet). Secure to the legs (the top of the stretchers will be flush with the top of the legs) using glue and 1-1/4” brad nails.
Drill pocket holes in each end of the support piece. Position in the center of the longer stretchers then secure using glue and 1-1/4” pocket hole screws.
Position the base frame on the bottom of the cabinet. It will be located 12” in from each side, and 3-1/4” from the front edge. Secure using glue and 1-1/4” pocket hole screws.