Locate the mounting brackets that came with your dishwasher. You should have a front bracket and two side brackets. These brackets may be rectangular or L-shaped. One side of the bracket attaches to the dishwasher and the other side to the counter.
Attach the side brackets to the dishwasher, at the top of the machine on either side. Screw the bracket to the dishwasher at the appropriate location, as indicated in the installation manual for your particular dishwasher. Screw the other side of the bracket to the countertop.
Open the dishwasher door. Near the center of the dishwasher opening, at the top, you'll see the location for the front bracket. Screw one side of the bracket to the dishwasher and the other side to the front of the cabinet.