It's very easy to challenge someone from the Topic Page. Please follow these steps:

Enter a topic page by tapping a topic icon. On the topic page you have 3 options next to the topic icon: 

  • Play
  • Follow/Unfollow 
  • Rankings

Tap the Play button and you´ll enter the challenge screen where you can either tap Play Now to matchup with a random opponent, or choose your opponent from your list of friends.

You can search through your friends in the search bar, or select a friend from the suggestions below. (see picture)