Slitherine

I am unable to find the challange tabs or a way to challange for a game. I need the basic way to do, I am a bit dense on this one, so detailed instructions would be great. am I challanging from the game itself or from the web site. I cant find the tabs in either location..thanks for the help

maybe better to post in the game section you wish to get or have help on tbh (to be honest)