1. Launch Word 2015 and open the doc you want to protect.
2. Go to TOOLS>PROTECT DOCUMENT.
3. A password protect window will open. You can password protect the doc at two different levels if you wish - one to open the doc, another required to modify it.
4. The icon for the doc will show a lock on it. You will need to provide the password(s) to anyone you want to allow to view and/or modify it, obviously.