Right on top of the page, you would see a "create a new hub" button. You click that if you want to create a new hub.
If you want to add an article in a hub that you've previously created, go to my account, and you'd see all of your previously created hubs. On the left side of the hubs, you'd see an "Edit" button. That button is the one that you click if you want to add an article in that existing hub.
I hope I answered your question.