Letters of recommendation may be submitted online by your recommenders at any time after you start your online application. It is strongly recommended that you waive or decline to waive your rights of access to your letters of recommendation (using the online application system) BEFORE you start inviting recommenders to submit their information for you.
How do I approach potential letter writers? First, make a list of professors and/or supervisors who will be your best advocates. Then, set up an appointment to discuss your request in person. Do not make the request via email. Be prepared to articulate your interest and reasons for attending graduate school. Letters of recommendation are written strictly on a voluntary basis. The best approach is to ask potential letter writers if they are willing to write you a strong letter. If you sense reluctance or the answer is no, ask someone else.