How do I pin things into Snapzines?

I saw this feature, and it looked useful, but when I looked it up and saw an FAQ, it said there would be a button, but for the life of me I cannot find it. Can someone explain to me how to use it?

Sorry if this isn't the right place to be asking this. I've seen a lot of users asking here though, so I thought it might be fine. If there is a place for these sorts of questions, could someone point it out to me?