Measure the depth of your cabinets. Cut your pine plank with the power saw so that it is as wide as your cabinets are deep. If you're not highly skilled at long, straight cuts, ask them to pre-cut the lumber at your home improvement center.
Drill pilot holes in your pine plank using a drill bit 1/4 inch narrower than the diameter of your screws. Drill one pilot hole in each corner, about 1/8 to 1/4 inch from the edge.
Measure the width of your cabinets.
Drill four additional pilot holes in your pine plank to correspond with the inside corners of your cabinetry.
Sand the pine plank. Your cabinets should already be reasonably smooth. If not, sand the cabinets as well. Wipe the sanded plank and cabinets with a tack cloth to remove the dust from the sanding and allow it them to air dry.
Stain the plank and cabinets to matching colors. Because they are made of different woods, this may require a different number of coats for planks versus cabinets.
Set up the cabinets so the outside edges are 48 inches apart.
Lay the pine plank on top of the cabinets so that its edges are flush with the outside edges of the cabinets.
Using one screw in each of the eight pilot holes, attach the pine plank to your cabinets. Your desk is complete and ready to be used.