There are two steps to this process. First, you need to make sure your book has reviews. Many of the sites that advertise Kindle giveaways require a minimum number of reviews (three, five, ten or, in some cases, more than twenty).
How do you get reviews? Send out a press release, well in advance of your book's publication, to any groups, online media, and reviewers who might be interested - including everyone you have ever known or met. Offer to send them a free PDF file, and make sure you have a dynamite summary that will get them interested. (Pester the people who have promised to write a review until they do. This is not the time to be shy.)
Now choose your free days on KDP Select.
Once you have booked your free days. Send out notices to these sites. (Read their guidelines now. Many require advance notice):
Author Marketing Club (join, and then use their free marketing tool)
It’s Write Now (fill out form on Author Marketing Club Submission Tool)
Free & Discounted Books (fill out form on Author Marketing Club Submission Tool)
The Kindle Book Review (fill out form on Author Marketing Club Submission Tool)
Pixel of Ink
Ereader News Today
Ereader Utopia
Free Book Dude
Book Goodies
Kindle Book Promos
Ereader Café
Awesome Gang
Frugal Freebies
Indie Book of the Day
Bargain Ebook Hunter
The Frugal Reader
Free Booksy
Ebooks Habit
Ebook Lister
Indie Books Promo
That Book Place
Orangeberry (3 reviews needed)
Ebook Lovers (10 reviews needed)
On the day your book is free, submit here:
Addicted to Books
Snickslist
And post on these Facebook Sites:
https://www.facebook.com/ourawesomegang
http://www.facebook.com/BookGoodies
https://www.facebook.com/KindleNation
https://www.facebook.com/eReader1.UK
http://facebook.com/freeebookdeal?sk=wall
https://www.facebook.com/kindle
http://facebook.com/pages/IndieKindle/106911752752245
http://facebook.com/pages/Kindle-Finds/217115528350246
https://www.facebook.com/EreaderNewsToday
https://www.facebook.com/pages/The-Frugal-eReader/101086513289732
https://www.facebook.com/getfreeebooks
https://www.facebook.com/FreeBooksDaily
https://www.facebook.com/FreeDigitalReads
https://www.facebook.com/pages/Free-Kindle-Books-Updated-Daily/155923931093850
https://www.facebook.com/TheeReaderCafe
http://facebook.com/iauthor?sk=wall
http://facebook.com/IndieBookLounge
https://www.facebook.com/JumpSeatBookClub
https://www.facebook.com/KindleFire
https://www.facebook.com/TheDigitalInkSpot
https://www.facebook.com/ebooksfreefreefree
https://www.facebook.com/digitalinktoday
https://www.facebook.com/KindleUtopia