Cut a sheet of particle board to the desired dimensions for the desk.
Trim the Formica sheet to match the dimensions of the desktop, using a heavy-duty blade.
Glue the Formica to the particle board. Carefully follow the application directions for the Formica glue. Some glues require clamping or curing time before use.
Attach the poplar trim to the sides of the desk surface that will not be flush to a wall. The trim provides solid visual appeal and protects the edge of the desk. The top of the trim should be flush with the surface of the desk. Use small finishing nails to secure the trim to the particle board.
Mark the location on the wall where the countertop desk will be mounted. Many desks are mounted between 32 and 42 inches, depending on intended use. Identify where the wall studs are located.
Mount the steel brackets to the wall. The brackets should be affixed with heavy wood screws sunk directly into a wall stud, to maximize the desk's stability. Use one bracket for every stud along the desired length of the desk.
Place the desk surface on top of the mounting brackets. Use wood screws to connect the desk surface to the brackets.