Hey Scarlett, On one of the last kickoffs of each game there will be a star on the field. If it is grey that means you need to collect it, if it is yellow that means you already got it. When you collect a star it will show on the tournaments screen that you have earned a star for that round. There is 1 star per round, so you can get a total of 3 for each tournament.
How do you collect the star