Producing : Soundtrack for movie by Tessa Mckenzie

Okay, I have a question what would be the process to contract a well known artist for the soundtrack for a motion picture. So, I’m trying to stay ahead of my steps. So, so far our music producer open the discussion they told us to sent them an LOI breakdown of what we offering. What come next in the discussion, how do we offer a well known artist to do a soundtrack. What else come in the discussion. Any intel would be greatfully appreciated. I know all about the movie, the points and all. But, soundtrack is a different beast.