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 can I go about getting good letters of recommendation? Since your best letters will come from those who know you well, make an effort to get to know your professors and/or supervisors. A few ways you can do this are to speak up in class, select courses with small class sizes, take more than one class from a professor, do research for a professor, take on optional projects, and regularly attend office hours. The best strategy you can use to get a good letter of recommendation, particularly if a professor hasn’t known you long, is to provide your letter writer with ample information about you. This way, you will get a letter that includes concrete details about you, instead of a letter that contains only your grade, which is of limited value.