Start with getting into the cat/cow pose and place a book on your back. Then, get into the regular plank position with the palm of your hands on the ground. The goal is to first do a push up, then get on your elbows and hold the plank as long as possible. After that, stand up so you are standing in an “L” position. That’s how you complete the challenge. Of course, you have to balance the book(s) on your back and never let them drop.
To make it harder, simply add as many books as you like.