In order to submit applications to any program you have to complete and certify the CV portion of your application. Any research, activities, jobs, AOA status, etc cannot be changed once you certify your application. For most people, the research part is the biggest hang-up because they may be waiting for notification about pubs or presentations that haven't been announced yet and want to include everything possible in their CV.
In regard to LOR, you list your letter writers in ERAS and then when the letter is sent to your Dean's office they scan it and attach it to your application under the appropriate letter writer's name. The only thing that you have to do before applying is make sure you list your letter writers and then assign them to the various programs you are applying to. You can change the assignments later and I think you can add letter writers after certifying your app, but I'm not 100% sure about that. Some programs have cut off dates in early-mid October and you will need to make sure that you get all of your materials to them by that date, but most still download until Nov 1st (of course like someone said before, the earlier the better because you never know exactly how they do their reviewing and you don't want to delay).
When you add a program to apply to there is a page where you check boxes to send various LOR, your transcript, USMLE scores, photograph of yourself. All of those things are uploaded to ERAS once you apply by whoever is responsible for them (Dean's office, USMLE) and are downloaded by the programs whenever the program decides to begin downloading applications (it is highly variable).
In summary, the most important thing is to get you CV complete because you CANNOT change that once you certify it and you must certify it before you apply anywhere. Hope this helps. ERAS can be confusing.
