Set your level up against the inside counter space and draw a straight line from one end to the other where you want the counter side sliding piece installed. Do the same on the other side of the cabinet, making sure that both sides are at the same distance from the top of the cabinet.
Set the slides against the cabinet and mark the screw holes with an pencil.
Drill pilot holes where the marks are and then screw on the bar.
Set the drawer partially into the cabinet and mark where the slide falls on both sides of the drawer.
Remove the drawer and set the mating slide piece in place. Screw the side piece into position.
Slide the drawer into the cabinet. The first time you slide it into place it will usually give you some resistance because you are setting up the wheels. After that, the drawer should slide smoothly.