There are 12 months in each level in career mode, and 2 tournaments every month. Theres a hard tourny and an easy tourny, its up to you which ones to play, i played the hard ones with the game difficulty set to easy and it wasnt difficult at all.
Keep an eye on the season leaderboard, in the tournament menu, because you will need to be 1st in the table to progress to the next season, if at the end of season you are not in 1st, you will need to play the whole season again.
You don't have to win every tournament, i skipped 3 or 4 each season depening on where i was in the table and how close 2nd place was to me
When you have completed the tournament in december the achievement will pop