First, ensure that the rectangular metal "tongue" or blade hinge assembly is in standing position or open. If it is closed, insert a flat head screwdriver under the tongue and applying enough force, push the hinge back to standing position. Once this is complete, slide the blade on the tongue, turn your clipper on and push the blade down and snap the hinge assembly closed. This will ensure the proper engagement of the blade on the driving tip. If you have any further questions, please call us at 800-830-3678.