This achievement just requires that you win a match in extra time. To get to extra time, the game must be tied at the end of the second half. You MUST be competing in an elimination game or else the game will just end in a tie and not proceed to extra time. Elimination games are those which the loser is eliminated from the tournament or league.
In extra time, you must outscore the opponent. This is best attempted on the easiest difficulty. If you have trouble scoring goals in a timely fashion, increase the game length so that extra time is longer as well. If the score is still even after extra time, the game proceeds to a shootout and you will not be able to unlock the achievement there.