Six months out you can request your retirement paperwork from HR. Complete the forms. Three months out you can request an appointment for retirement counseling. It took me 5 weeks to get an appointment. At that time someone will call you and go over each page of your paperwork. Don't skip this step. It is very helpful. You send this into HR, you get your retirement date and yippee.
Are you asking before heading out the door or after? If before I sent for my blue book, had a local former HR person go thru the book with me on a Saturday, mailed the book in on Tuesday, and retired on Friday. If the question is for after you retired it can take up to 6 months for OPM to complete the adjustments to your retirement pay.
I retired 12/2017 . Got my first check in a month. I had prepared myself for the 6 months but was still living on my last pay check . There are videos on you tube that literally walk you through filling out the blue book. Also when you get the call they also walk through the whole thing