Position the kitchen cabinets so that their outer sides are 68 inches apart. Try to find cabinets that are around 28 inches high, for the proper desk height, around the same length deep, and around 20 inches wide—many kitchen remodeling companies sell kitchen cabinets pulled from kitchen jobs very cheaply.
Center the wood sheet desk top over the cabinets, with an inch lip over all sides. Trace in the position of the cabinet corners on the underside of the desk top.
Miter one end of each of the 16 planks to a 45-degree angle with the saw. Arrange the planks into eight corners, with the mitered ends joining to form a right-angle. Glue together at the mitered joint.
Take the desktop off the cabinets, and lay flat, underside facing upwards. Place the corners on the outside of the corner lines traced on the underside of the desktop. Drill in place.
Replace the desktop on the cabinets, fitting the cabinets snugly inside the corner holding planks.
Sand the desktop, softening the edges, then paint or varnish as desired.