I don't know much, only what I have heard from my folks.
In my state, and many others, you go onto a government website. First they ask you lots of questions. After you finish the survey, it informs you to wait for the server to process your turn, and this can take 20 minutes to an hour. Theirs kicked them out because too many people were online trying to sign up. (Mind you, they are senior citizens, and he told me this later, because he thought he was using the computer wrong, just doing the "easy sign up" like the news told him to do.) If you succeed in getting an appointment, it gives you a number in the thousands, representing how many appointments in the queue have to be scheduled before you.