Pick an interesting crime. Read newspapers regularly and find one that has a lot of good elements that will hold your interest and that of your readers.
Start investigating. Interview people who were involved in the case--from victims to family members to the police. Read everything that was written about the case.
Build a relationship with the law enforcement officials who are involved in the case.
Attend the trial and speak to everyone you can about the criminal, the victim and other characters in your novel.
Sit down and write. Now it is time to tell the story of the crime. Hopefully you will find most of the information you need in your copious notes--if not go back and get the answers you need. If you want your true crime novel to be believable, you can't fudge the facts.