The day before, you want to prime your audience. Let them know something big is coming. If you want to share that you’re going to be in a promo, that’s okay. If you want to tease, that’s fine, too.
The days of the promotion, you blast your newsletter and social media with the graphics and link to the cross-promo landing page (supplied by MM Book Buzz). Only do this once or twice per day. Encourage everyone to share. Make sure your audience knows what genres are available in the promotion as these promotions are genre-specific.
During the days of the promotion, you should see a spike in sales (if you’ve priced your promo at 99c), free downloads (if you priced at Free), and KU page reads (if your book is in KU).
Over the week or so after the cross-promotion ends, you should see a boost in sales / downloads / KU page reads for the rest of your back list as new fans plow through everything you’ve written.
Note: These results are typical, but not guaranteed. It depends on many factors, such as each author’s fanbase size, author participation (whether the authors all remembered to blast their fans with the deals), how good their cover and blurb are, and more.