French cleat should be attached to the wall by screwing through the cleat into wall stud or blocking using the fasteners provided. The cleat should be level and at the indicated height.
Use 3” wood screws to attach lower, loose part of French cleat to wall with the small side touching the wall as pictured.
The lower cleat that attaches to the wall is usually loosely attached to the back or inside (unfinished) of the cabinet with one or two wood screws. Remove cleat and mount to wall. The cabinet then hangs, or rests on the wall-mounted cleat as depicted.