i suppose that you bought a used mower. some of those cut left hand, but most cut right hand. it will depend upon that. you have one long belt. some mowers have a belt on the deck and a drive belt.
start at the pto pulley. place the belt on the pto pulley and then under the idler pulleys and then to the center pulley on the mower. go around the the center pulley, (lowest groove) either left or right, depending upon which way the mower cuts. and then to the outside pulley, then back to the center, one grove up, then to the other pulley and then back to the center. then tighten idler pulleys. tighten belt really good, snug it up. make belt tight.
maybe i have confused you really good.