How DO I Fix This Problem?

i have a sears craftsman riding mower,older model no.917.252520. this was running and cutting but the blades were wore outand just layiny the grass down on one side. we took off the deck and put two new blades on, put the deck back on. made two swipes and the drive belt came off, hadn't before but we put it on and it kept coming off. bought new belt put it on and it still came off.bought new tension pulley (old one was broke)put it on,went to start it then the key switch quit. bought new switch put it on , now it still wont start. i need a wiring diagram i guess. what do i do? it ran fine untill we put on new blade.