To participate in this opportunity, it is important for the author to have a story that is already written and ready to go. The story should read like a feature or author interview, not an ad. That means that all opinions should be put in direct quotes and attributed to someone. Instead of saying "this book will change your life" like an ad would say, a feature story would attribute it as someone's opinion--"This book changed my life," said Joe Smith.
The story should contain all the information the reader needs to know before making a purchase; what the book is about, the author's motivation or reason for writing it, the author's qualifications or past publications, the average star rating on Amazon or Good Reads, quotes from reviews, and the purchase info of price, binding, ISBN number and where to buy it. The story can also include the author's social media sites such as a web site, Facebook page or Twitter address. If the story is ready to go, most bloggers will edit it to suite their style, and appreciate the ease of posting the article.
The author then finds bloggers that are willing to swap stories. One place is on Linked In, in the Book Promotion group. Triberr, a service for bloggers who want to expand their reach on Twitter, also has a discussion group for bloggers who are seeking guest posts.
It's a simple "you rub my back, and I'll rub yours" trade. It's best to find bloggers in the same subject or genre as your book, because that is where your customers are. An author who blogs about science fiction is not the best place to promote a romance title, but a blog about relationships and dating would have readers who are interested in falling in love.