Bella becomes a vampire just after giving birth to Renesmee. The reason was that she had lost blood, Renesmee had scratched her inside skin and also her baby had broken her bones. In Jacob's point of view, it's chapter 18. In Bella's, it's chapter 19 of Breaking Dawn.
Bella becomes a vampire after she delivers renessme in Breaking Dawn. As soon as Edward hads renessme to Rosalie; he then injects venom into her heart with a needle. Then bites her throat, wrist, and crease of her arms to inject more venom into her system. All the while Jacob performs cpr to spread it.