Generally 5 Guides are required;
These should not be runescape related and your chances of joining the team increase as the quality of your guides do.
Like Pen or Eric said. "Quality Not Quantity"
Write guides that would fit into the appropriate sections of the User Education Section. Don't just write all your Guides on one Subject, Try and alternate.
When you have done that Write a paragraph on why you want to be a User Educator. Post your application in the correct section, and Wait to be rated by The Head User Educators.