how do we earn from google

Yes, you could, if you had a website that a lot of people visited. I would much more reccomend a Pay Per Imperession thing, although the popups would be annoying, if your site had enough of a hook for people to keep coming even despite the popups, you could make up to 1.20 CPM or 6.00 UCPM, which is really good compared to a lot. If you want the URL, send me a private message on here.

how exactly?

None by my count!