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.
Inform yourself about the person you are recommending. Spend some time talking to them about their goals and the position they are applying for. Gather together their current resume, any memos or notes you may have on them, and any other information that will help you as you write the letter. The best recommendations are thorough and specific, and having all the information at hand will make the process much easier.