My son wrote a letter to each teacher he was asking for a letter of rec, explaining what he was doing and how he would be honored to have a LOR from them. He went to each teacher on the last day of his junior year, explained to them his future plans, and gave each a self addressed large envelope to mail them back to him in (for noms). He also explained in the letter that the rec letters would be done electronically for USNA and USMA and that it might help to look in their spam folder if they didn't get it by x date. Just a heads up: One of our MOCs required a Senior math instructor LOR, so he had to wait until after senior year started, and got to know this teacher before asking for a letter from him.