Self explanatory, you must complete a season without simming any matches. That mean that you must play every match (Including Cup Games etc.) from start to finish.
If you want to do it as quickly as possible, choose Sydney Fc as your team when you start. (Sydney FC are in the Australian A-League). The Australian A-League is the smallest league on the game therefore it'll be quicker to do than the Premiership for example. It only has 25 league games. Quite time consuming but keep at it and you'll have it quicker than you'd expect.