404 page template element

Hi. I have looked at the other help topics on this but still didn't quite understand. I am looking to make my own 404 page and I have saved a draft element but not published it yet as I want to make sure I've done the right thing.

I created a new element - block. Then chose 'content template' under the settings and 404 template under location.

If I hit publish will this replace the current 404 page with what I have in this element?

Also where do I go to see the 404 page live?

thanks!
Kristen