Sand and varnish the desk top if it needs it. Desk tops can be purchased from many home and office stores; you also can use a piece of timber, or even recycle a wooden door to use as a desk top.
Choose two filing cabinets that are around 28 to 30 inches high. Place the desk top on the filing cabinets, which should be positioned so that the top has a 2-inch overlap on either side. Trace the outline of the cabinets onto the underside of the desk top.
Cut the 1-by-1 lumber into 16 lengths of 5 inches. Miter one end of each piece of 1-by-1 to 45 degrees. Mate the mitered ends of two pieces of the lumber to form a 90-degree corner and glue together. Repeat with remaining pieces so you have eight corners.
Lay the desk top upside down so the traced cabinet positions are visible. Place a mitered corner over each of the traced corners and screw them in place with the 1 3/4-inch screws.
Position the cabinets where you want the desk to go, then place the desk top over them. Each cabinet should fit snugly within the wooden corners on the underside of the desk top, holding the top in place between the cabinets.