You can back a project by clicking the "Back This Project" button at the top, or by selecting a reward. We try selecting a reward first.
When you tap on a reward, you can edit your pledge amount before continuing. We try to add a pledge amount that's less than what's required for the reward to see what happens. As expected, we can't continue.
Now we scroll back up and try the flow with the "Back This Project" button. Tapping on it takes us to a page where we can either select a reward again or make a pledge without one.
Now we choose our payment method and complete the pledge.
The pledge has been made. The "success" page has a couple of big share buttons on it for sharing on Twitter or Facebook.