This one can be a bit tricky, depending on your skill level. Start a casual match, then choose your team and opposing team. When you've finalised your choice you'll be presented with 3 challenges, e.g win the match without performing a substitution, make a diving try for every try scored, make 3 interceptions etc.
If you feel cconfident in completing the challenge(s) play on and do your best, if you get one that you feel is too difficult, just quit the match and start again and you'll be presented with 3 new challenges and try again.
Good luck!