After you have accepted a challenge or when you are sending a challenge, you can choose your lineup. All you have to do is click on one of the player silhouettes (goalkeeper, defender, midfielder, or striker), then you will see the fixture list where you can choose the match and finally the player you want to choose for this position. Do the same for the remaining three player silhouettes and then you lineup is complete.
NB! If you hold down on the player you want to pick instead of clicking you will be presented with a statistics overview for that player which might be useful before picking the player - swipe to see the other players on the same position before making your decision.