Essentially, you read the selected book once a day for 5 days in a row. After each reading, you do a couple of activities that somehow relate to the story. The Before Five in a Row book lists many great activities for each book but I've also collected many more on my Before Five in a Row Pinterest board.
To plan, I start with locating the books that we will read. Many of them can be found at our local libraries. Others I buy used at library book sales or from friends whose kiddos have outgrown them. All of the suggested books are really great pieces of children's literature so I do try to buy them when I can so that we can read them over and over at anytime.
I then look through the books and decide on the best time of the year to read them based on things like if the story takes place during a particular season or if there is a theme to the story that ties in with something else we will be doing later in the year.
Here is a Before Five in a Row Book List (PDF) that I made to help me keep track of all of this:
To help me plan the activities we do each day, I created this weekly planning page that I fill out for each book. It lets me keep track of which activities we will do on which day, the subjects covered, and which supplies I need to have on hand.
Here is a PDF version if you'd like to download it: