You send and email to the PA/PR person. In our experience it always begins with an email. Often it goes un-answered and you look for other routes. Finally you get through. Have your story ready and use your contact with the Subject, if you´ve been lucky enough to have one, as an opener. After preliminaries the PR/PA gets down to the nitty-gritty and sends a list of demands. Read it carefully and comply if it doesn´t dilute or destroy your plans for the project. For example: if PA/PR demands that Subject has the right to check all footage tell him "NO!"
"Chasing Esa-Pekka" was a long hard slog because his "people" wanted him to have control, and veto power, over everything that went into the doc. We finally called a halt and said "Sorry but we´re not doing this documentary under those conditions." They even contacted our commissioning editor at the Finnish Broadcasting Company (YLE) to back them up. But our CE supported us. Although we did have to take out the F word, which made one scene human and funny, to keep them happy. We never heard what they thought of the doc, although they later asked permission to use it. We gave it to them with pleasure.
Lesson 38: No matter how famous the Subject is, know when it´s time to say "NO!"
next week; 38 The Right Gear