Everyone is different, but I would recommend at the very least doing an outline:
Do a mind map of the subjects you want to touch upon and the most important events.
Decide in which order they occur.
Take the events and make a new mind map for each of them.
Convert the mind maps into a timelined plan.
Voila, you've got a timeline to rely on. Details of HOW things happen (except when the "how" is important enough to count as a major event) and the things people say to each other will be decided by you when you write it out later.
Now whether you want to write it out right away (i.e. the plotting route) or leave it for when the time of necessity arrives (pantsing) is up to you!