Chart the structure of the book you are reading. Who are the important characters? What is the setting(time, place)? What is the basic plot? What problem(s) is central to the plot, and how is it resolved? Do the important characters get what they want/need?
There are some fun printable pages on Pinterest (and thus all over the Internet) for this sort of basic book report. Here’s one that might be helpful for this challenge, though if you browse Pinterest or some of the sites these graphics are posted on, you may find others you like better. Or, make your own version. Many of these printables are also designed to be colored, so pull out your crayons or markers and have fun decorating your book report.