Remove the legs of your desk. Flip your desk over and locate where the legs are attached. There will be screws traveling parallel to the desk top connecting the legs to a side joint. Use a phillips screwdriver (or the tool that is needed for your desk) to unscrew each leg from the desk.
Measure each leg with your measuring tape to determine which is the shortest and causing the unbalance. The length of your shortest desk leg will be your new marker. Mark the length of your shortest desk leg on the remaining legs with your pencil.
Make your markings near the foot of the desk legs where you will cut. Measure the legs several times making a pencil mark on every edge if the desk leg is square or octagonal or at different points along its diameter if the leg shape is round. Multiple marks will ensure that your measurements are correct and that your desk will not be cut uneven again.
Cut the three marked desk legs with your chop saw. A chop saw cuts with a single blade through a single plane creating a flat and even surface after the cut. Make sure the desk leg is on an even plane while cutting. You can check this using a level. This will ensure an accurate cut.
Attach the legs back onto the desk. The legs are now all the same length and the desk should be level.