As the description says, you need to complete a game in the Ashes mode.
From the main menu, select Ashes Mode and select whether you want to play as Mens or Womens teams and then either Australia or England.
Once the game has loaded, play through the first test, which will normally consist of both teams batting. If you want this to go a bit quicker, you can declare on both your batting innings (Pause > Tactics > Declare) this will end your innings with whatever score you have earnt already.
I recommend playing through though as there is a lot of Achievements that can only be won by playing Ashes Series and will cut down on any grind you have to do at the end for the 2 biggest achievements (200 wickets and 2500 runs)
Hope this helps.