How To Hook A Breaker Into A Box?

Usually (not always since there are many types of breakers and boxes) the back part (where wires hook up) of the breaker goes in first and the rest of the breaker swivels back and is snapped into place and locked in in the center.
Be sure to turn the breaker off before attempting or the back part of the breaker where wires hook in will be hot.  It is really best to turn off the main breaker for most safety.