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.
Target the recommendation. Is it for an academic post, a job, a volunteer position, a background check, or a personal reference? Write your letter so that it is focused on this purpose. For example, if the letter is part of a job application package, it should focus on the professional qualifications and conduct of the candidate.