The short answer is that you will receive a Letter of Instruction (LOI) during your M-III year that will spell all of this out.
I will caveat my answer by stating that I am speaking from the experience of Army HPSP.
You must submit your application by October 15. Your application consists of your ERAS application, your Pre Accession Form (located on HPSP FYGME website you will be allowed onto once you receive your LOI; basically a form that is already filled out with your name, address, physical exam info, etc. that you sign off on), Preference Priority List (where you list the specialty you wish to match into and the order of your location preference; basically your match list), Physical Exam, and Deferment Form. You can submit all of this information anytime between Sep 1 (the first day you can submit ERAS) and Oct 15. After Oct 15 only the order of your preference list may be changed until Oct 31, after which no further changes can be made.
You must schedule your interviews on your own - this is not done through ERAS as it is done in the civilian match. Contact the program(s) and schedule the interview. You should strive to have your interviews completed by Oct 31, but I imagine that you can interview up until Dec 14 if you so choose and the program is willing.
If you would prefer to match at a particular program, it is advisable that you do a rotation there and do your interview during that rotation. You can schedule your rotations as early as Jan 1, but they must be scheduled by about May 15 if you wish to do them while on orders (ADT). If you are doing your rotations on your own dime and not on active duty, you must get a memorandum of agreement (MOA) between your school and that medical center. It could take upwards of a month to get the MOA approved so you need to plan for this in advance.
Match day is on or about Dec 15. You will log onto HPSP and find out where you've matched. I don't know much about what happens after that (other than that you must withdraw from the civilian match and sign some documents), so someone else will have to take over.
HTH.