Follow these steps for the How to take a screenshot on a Chromebook

Using Keyboard Shortcuts

For instant access, press the keys Ctrl + Show Windows key.

( In case, your Chromebook has no Show Windows key, it could be marked differently or substituted with an F5 key. In that case, use Ctrl + F5.)   

Selected Area Screenshot:

Bookmark Ctrl + Shift + Show Windows key.

If you don’t have the Show Windows key on your Chromebook you can use the shortcut Ctrl + Shift + F5.

Your cursor will change.

To capture your desktop, use your mouse to left-click and then drag the mouse to create the shape of the capture area you want.