Measure the width and height of the space in the desk where the drawers will be installed and select drawers that will fit in that space. Add 1 inch to the measurement for the width of the drawer slides. Determine whether you want to use shallow drawers and have extra space or use deeper drawers that will fill the open space completely. Use a pencil to mark on the desk the location of the top and the bottom edge of each drawer. This is where the drawer slides or supports will be installed.
Expand a drawer slide to its full length and separate the two pieces of the drawer slide. Depending on the kind of drawer slide you may have to push, pull or depress a piece of plastic on the slide to get the pieces to separate. The smaller piece attaches to the drawer and the larger one, sometimes called the track, attaches to the desk.
Attach the smaller piece of the drawer slide to the drawer. This piece usually has a bent metal piece to indicate the front. Hold the slide flush with the front and the bottom of the drawer. Use a screwdriver or a drill with a screwdriver bit to drive the screws that came with the slides through the holes in the slides and into the drawer. Separate the pieces of another drawer slide and attach the drawer piece of the slide to the other side of the drawer. You should now have one drawer with two metal drawer slides attached.
Measure the distance from the top of the drawer to the top of the drawer's slide. Mark where the track will need to be installed on the inside of the desk space. Make sure the pieces on the drawer will fit into the tracks and the drawer will open and close properly. Hold the slide to the desk where it will be attached and use the level to make sure the track is level. The pieces of the slides must line up for the drawer to work properly.
Install the tracks on the desk using a screwdriver or drill with a screwdriver bit. Install just the pair for the top drawer. Slide the drawer into place and check that it moves smoothly. If not adjust the placement of the slides as needed.
Separate the second set of drawer slides. Install the slides parts on the second drawer following the same directions as for the first drawer. Insert the second drawer into the desk slides and check for alignment. Adjust if necessary.