I'm going to verbally say smh my head instead of actually shaking my head
That's an odd thing to do. What is the reasoning behind this?
I'm not really familiar with the feeling of shaking my head smh
If you tell people smh you might confuse them. It is better to have non verbal smh motion than confuse people who could misinterpret it.
Why do I care? I'm going to use it anyway.
You are going to really make people have a tough time understanding you. You should stick to normal English statements and actions if you don't want people to avoid interacting with you.