The following presentation takes you through the steps for writing the Extended Essay.  Your supervisor should let you know your school's internal deadlines for each of the components: note-taking, introduction, body, discussion.

It is important that you meet with your supervisor to discuss any problems you may be having before it gets too close to your final deadline.  And remember, your supervisor may only give you one set of feedback on the rough draft, the stronger your first draft, the most likely you will get a better final grade.