I have a Craftsman 6.75 horsepower self-propelled front drive lawnmower that the belt keep slipping off I put a new belt on at the beginning of this season and it seemed to be okay but it started slipping off and I'm not sure how to assess the issue I'm not sure how to tighten it I'm not sure if something is worn out i do not know how to address the troubleshooting. I dont know how to use the tensioner on the handlebar nor what else to look for as a possible cause.