How do the Volturi attack The Cullens to obtain Renesmee?

They really didnt attacked them 2 obtain Renesmee, they just got together all the Vulturis and placed a dag 2 go 2 the Cullens house.
Of course they were thinking Renesmee was dangerous, that changed when Alice and Jasper showed them Nohuel

They did not, the Volturi failed at the job because of Bella's knew Powers and the help of other Vampire and Wolves

They didn't attack to obtain Renesmee. It was Edward and Alice that Aro wanted, of their powers, for his "collection" in the Volturi guard. The Volturi just used Renesmee as an excuse, because they had been mistakenly informed that she was a human child who'd been turned into a vampire. They would have fought the coven, executed the rest and Aro would claim that Alice and Edward's thoughts were repentant in order to spare them. Then another member of the guard (Chelsea, I think her name is) would use her ability to change Alice and Edward's loyalties, making them willing to kom bij the guard.