Cut your lumber to the needed dimensions with your power saw. Do this by drawing a line across the lumber a distance from one end equal to the length you need that piece of lumber to be. Start the power saw and move the blade along the line you drew.
Draw lines on your drywall to match the dimensions you need. Cut the surface of of the drywall by inserting the blade and running it along the lines you drew. If the blade doesn't go all the way through the drywall, you can break it by bending it along the cut instead of making a second cut.
Sand all surfaces of the planks with coarse grit sandpaper. Sand a second time with fine-grit sandpaper on one face and all edges of both planks.
Stain the sanded surfaces of both planks. Make your brush strokes run parallel to the direction of the wood grain, brushing in one direction only.
Set two 72-inch beams and two 39-inch beams in a rectangle with the shorter beams inside the longer beams. The beams should rest on their narrowest sides. Nail them in place with two framing nails per joint, driving them through the face of the longer beam and into the end of the shorter beam.
Set two 36-inch beams and two 15-inch beams in a rectangle, all beams on their narrow sides. The shorter beams should be inside the longer beams. Nail them in place with two framing nails per joint, driving them through the face of the longer beam and into the end of the shorter beam.
Set up the three rectangles so they form the shape of a "C," with the one smaller rectangle at each end of the larger. The ends of the smaller rectangles should abut the face of the larger rectangle. Nail them in place with three evenly spaced framing nails per joint. Drive the nails through the lumber of the smaller rectangles and into the lumber of the larger.
Set the 42 by 36-inch sheets of drywall on the front face of the framed desk, the face without the smaller frames attached. Nail them in place using the drywall nails, one nail per 8 inches all the way around the frame.
Set the 36 by 18-inch drywall sheets up against the outside face of each smaller rectangular frame. Nail them in place using the drywall nails, one nail per 8 inches all the way around the frame.
Install a 42-inch drywall corner at each front corner of the desk, nailed in place with three drywall nails per face.
Install a 36-inch drywall corner at each outer corner of the desk, nailed in place with three drywall nails per face.
Run a length of drywall tape over all four drywall corners and the seam at the middle of the desk's face.
Apply compound mud over the taped sections of drywall. If you want to add drywall texture, do so at this point. Allow to dry.
Set one of the planks, stained side up, on top of the long rectangular frame. It should overlap the customer side of the top beam by 3 inches, and the receptionist side of the top beam by 11 1/2 inches. Nail it in place with a line of finishing nails at the center of the beam at 6, 30, 42 and 72 inches from one end. Drive the nails through the beam beneath and into the underside of the plank.
Set up the second plank so it rests, stained side up, with each end on top of the shorter rectangles. Nail it in place with two evenly spaced finishing nails per side, driven up through the beam beneath and into the underside of the plank.
Lightly sand the front of all three faces of drywall, just enough to smooth the texture out.
Paint the faces of drywall. Allow to dry.