How do i put back a serpentine belt on a 2012 Ford focus 2.0L diesel

there should be a belt routing diagram under the hood some place. a sticker on the fan shroud or somewhere right in the front. you will need a wrench, some use a 3/8 drive square head and some use a regular bolt head. this will be to loosen the belt tensioner. once you have the belt routed you will loosen the tensioner and slip the belt back on.

/The bigger question, is why did it fall off? better check all of the pulleys to make sure one of them isn't bad. the car seems pretty new to have a pulley bearing go bad, but anything is possible.

Everything Mike said above is sound advice, and to reiterate what he asked, why did the old belt fall off? Check that the idler pulley and belt tensioner pulley are good. Check the other accessory pulleys (alternator, power steering, AC compressor) are also good and none have any wobble or bad bearings. If you have a high-mileage motor, you may want to replace the belt tensioner and idler pulleys along with your new belt, they're cheap and it's good insurance - and you're already into that area of the engine.

he didn't ask that. If you'd like to throw all that unsolicited information fine, but answer his question first.