To follow another user on Clash, just tap on their profile pic/name to enter that user's profile page. Once on the users profile just tap the "+ follow" tab to follow that user.
It's that easy!
If you no longer wish to follow a particular user, simply return to their profile page and tap on the horizontal ellipsis (3 dots) found in the top right of the screen. You will now see a few options displayed, tap "Unfollow" to remove that user from your followers list.