Get in a push-up position with your shoulders directly above your hands.
Now lower the forearms and elbows down toward the floor. Your biceps should make full contact with your forearms at the bottom.
Come back up.
I Can't Do It!
Yeah, this isn't as easy as it looks! Most don't get it right the first time. To make it more doable, start from the bottom with your forearms resting on the floor before you push up. To decrease the difficulty, pivot from the knees instead of the feet.
It's Too Easy!
To make it more challenging, elevate the feet onto a step. If your wrists bother you with this exercise, try positioning your hands on the edge of a step.
You probably never thought about it, but most people can't produce as much force using two bilateral limbs to perform an exercise as they can if they perform the exercise with each limb individually and then add together the force of each side. It's called the bilateral deficit and you should take advantage of it to pack on some muscle.