For Web:
While in your 'Matchup' page, you will see a button in the top right corner that says 'Contest', selecting this will bring you to the 'Contest Screen'.
For Mobile:
While in the 'Matchup' screen, tap the OwnersBox icon in the bottom right corner.
This will open the menu and you can tap the 'Contest' icon.
You can also access the 'Contest' from your 'Dashboard' by tapping the header of your desired matchup.