LOR requests will come directly to their inbox.
It is a very confusing process for the letter writers. Lots of requests from different people, different offices, different ways of submitting. Once people have agreed to write a LOR, I would first get their preferred email to receive a request and their preferred phone number. The next day I would hand-deliver and once home from school, email a short "Thank you for agreeing to help" letter with a very brief explanation of the different SAs to which you are applying---like full name and city where the SA is based, your bullet-point reason/s for wishing to attend, and your resume with accomplishments. Make it as easy as possible for them to help you.
If going for an ROTC scholarship, let them know that is different. Many people do not know the course of getting into a SA nor the difference betwixt SA, ROTC, enlistment. Also, let them each know that it will be a LOR from the MOC and a scholastic aptitude type of evaluation from the SAs, and each one is different.
My experience was requests went in this order
2 Senators
1 District Congressional Rep
SAs based on which application filled out first
NROTC
and non-SA scholarships went in order applied
So, no matter if you are applying to ROTC program and all of the SAs or just one SA, you are asking a lot of them. There are LsOR going to State Senators, local Congressman, and filling out evaluations of scholastic aptitude. All have their own evaluation forms.
My school district uses an on-line repository (ours is called "Naviance"). Students can choose whether to request a viewable or non-viewable to student LOR. An email request for LOR is initiated from there and students can later choose to send completed LOR to other scholarships.
Once all deadlines have passed, follow up with a "thank you for all you did to help me" handwritten note, and keep them posted as you find out your status.