There is no really good method to do this. Most can be worked around if the document is still in DOCX format.
It's not perfect, but I find it far more effective to save as PDF and send that out. There are some who have PDF editors and know how to use them, but far fewer than the number who have MS-Office and know the tricks to get around protection.
If you really insist on sticking with DOCX you can use ALT+L,PE (Layout Ribbon, Restrict Editing), then navigate to the checkbox under Editing Restrictions and check it. After it's checked, a dropdown box of options becomes accessible, the default is No Changes (Read Only), which is what you want. Then navigate to the button, "Yes, start enforcing protection," and activate it. At this point a dialog box comes up allowing you to choose password protection as the way to be able to unset editing restrictions or User Authentication. Most use Password Protection, which makes you do the usual entering of the password you want in two boxes. Then hit the OK key and the document can no longer be edited, even by you, unless you go back and turn off editing restrictions.
Brian - Windows 10 Pro, 64-Bit, Version 1909, Build 18363
Tolerance is the positive and cordial effort to understand another's beliefs, practices, and habits without necessarily sharing or accepting them.
~ Joshua Liebman