How can I add some words right under my custom header 'Ms Dynasty'?
There is more than one way to do something like that. Here is one possible way:
content: "Put your text here.";
You can play around with the text, numbers, and alignment values to change it.
Another way to do it would be to turn the title and tagline back on in your Appearance → Header page and then use CSS to move those to different locations using CSS. That way, you could use the main Settings page to change the text out by editing the Tagline and that would probably be an easier way to change the text often.