Measure the space for your built in desk. Measure both width and length and determine how big you want the finished desk to be.
Choose a style of built in desk. Simple styles include joists attached to the wall with a piece of wood or counter top across the top, to more elaborate designs with shelving and drawers below.
Mark lines on the wall or inside the alcove for the top of the desk and the brackets or joist underneath. If you build in to a very long space, you'll need one bracket on either end and at least one in the middle for good support.
Nail furring strips to the wall around the space the proper height from the floor and check to see if they're level.
Cut a notch in each bracket for the furring strips and nail to the wall at the predetermined locations.
Cut your counter top or wood to the right length for the space. If using a finished counter top, use masking tape and tape along the cutting line before sawing. This helps keep the counter top from cracking as you cut.
Lay your top across the brackets and furring strips and secure with nails.