Profiling serial killers - a biography

Thank you for popping in.

I asked this question about true crime and now have the same question about biographies: do you create a proposal and try and sell it to an agent, or do you write the entire biography first?

Just so you know I didn't trick you into this thread: my potential subject is a former FBI agent, a profiler on serial killer cases, and the David Koresh "incident."

Thanks all. This is kind of an old thread and I have it all figured out. But thanks for taking the time to respond, anyway.