I am indeed in my deferral year now
Best decision I've made -- if you're seriously considering one and don't have concrete reasons not to take an extra year, I highly recommend it. This is one of the last chances to have a truly "responsibility-free" year for a rather long time, so if you have things you'd like to do that require larger chunks is time, is a fantastic opportunity.
I've been doing a mix of productive and non-productive things and feel spectacularly rejuvenated and ready to get back into the swing of things. Compared to how I felt this time last year, it's a world of difference.
In terms of the process itself, at my school, I just had to send a one page letter outlining my plans for the year and why I thought it would be beneficial for me. It was pretty straight forward. Since then I've had to send back a signed form in late fall saying I was indeed planning on re-matriculating (the options were re-matriculate or request another year) and recently submitted a blank AMCAS deferral application to make things official.
Feel free to PM me about the details or if you have any questions
Edit: as others have mentioned, each school had different standards for what they'll allow a deferral. My school was pretty lenient; others state on their websites that it had to be something more significant (eg. Rhodes or some other time-sensitive grant). I'm sure admissions will be willing to give you an idea re: how strict they generally are if you shoot them an email.