I need help changing a belt on my Craftsman riding lawn mower.The belt that drives the blades.Ive tried many times and for some reason cannot do it! HELP ME!

Do you have the blade engagement engaged? I have removed an idler pulley, put the belt around it, and push it over and back down onto the pulley bolt, or pushed the bolt through the hole. Also, put it onto the deck first, then put it onto the pto pulley on the engine.