If you’re using OneNote 2016 and you’re trying to take notes from another screen, say a web site or a spreadsheet, press Ctrl-Alt-D to dock OneNote to the right side of your screen. This makes it easy to take notes while you continue to work with the notes automatically saved as you go.
Bonus – Click on the chain icon in the upper right to enable Linked Note Taking tied to the browser if you are taking notes from the web.