Clarify exactly what the publisher who hired you and the subject of the project is looking for. Although you may be a writer, you're being employed to do a job and will need to deliver what you're being paid for. It may be the case that your publisher wants certain parts of your subject's past left out of the finished book. On the other hand, you may be asked to deliver a warts-and-all account of a colorful life.
Study the sport played by the subject of your work. Publishers will usually choose a writer who has at least some interest in, and knowledge of, the sport played by the individual she's going to be writing about. As many sports people need to be wholly consumed by their sport to reach the top, the more a writer knows about the game her subject plays, the greater her ability will be to paint an accurate picture of the individual she's writing about.
Familiarize yourself with the history of your subject's career and personal life. Look back through newspaper and television coverage online or visit clippings libraries. Build a multi-dimensional picture of your subject and try not to focus on any one area of the subject's life unless you've been instructed to do so by your publisher.
Talk to the friends, family and colleagues of the sportsperson you're writing about. The subject of your manuscript should be happy to put you in touch with people he's worked with and those close to him. Conduct taped interviews that you'll be able to use later.
Construct a list of detailed questions from the research you've done so far to ask the subject of your work. You'll want to secure as much time as possible to get to know him and record his version of events. Make sure everything is taped for future reference. Arrange further access so you can fill in any gaps that arise. The more you can get the better. A ghostwritten sporting memoir will require scores of hours of recorded material.
Put the first draft of your manuscript together from the research, notes and recordings you've accumulated. Stay in close contact with your publisher and the subject so you can iron out any issues as they arise. It's a lot easier to discuss any problems at this stage rather than after you've submitted your first draft.
Submit your first draft and work with the publisher and the subject of your manuscript to arrive at a version all parties are happy with.