Like uglyboy said PRACTICE, PRACTICE, PRACTICE. You'll be glad you learned how to do it!
He's also correct that on a two wheeled cart (or manure spreader, or hay baler, or anything of that nature to start the turn you steer so that the tractor starts in the opposite direction you want the implement to go. Want the cart to go left? Begin to maneuver the tractor right (but just a little). Once the implement just starts to turn then keep the turn gentle by turning the front wheels of the tractor back toward straight, maybe even a little more if necessary. Best way to start is to practice backing in a straight line out in the middle of the yard. You'll learn how to make little corrections to keep going the way you want.
Oh yeah.... if you have 4 wheel steer, turn it off while backing.
Now... 4 wheeled "wagons" are just the opposite. When backing the tractor must start to move in the SAME direction you want the wagon to turn. They're generally more difficult to back up.
Have fun!