Summarize this article:

Out of the box, some Chromebooks have their battery detached from their body and in a separate plastic bag. Simply take the battery out of the box and remove the plastic bag. If your Chromebook's battery comes preinstalled, you can skip this part. Do this by sliding the battery, with its pins facing toward the laptop, into the battery compartment in the rear of the computer.

Summary:
Take the battery out of the packaging. Insert the battery into the Chromebook.