You add meta tags to the <head> of your HTML document under the <title> tag. For example:
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="description" content="Briefly describe your page content here for search engines to see....">
Manually adding them is going to be the fastest way, but you can also go to Insert > HTML and choose from the list starting with "Meta" (in the middle of the context menu) if you're not 100% sure of their structure.
Those options will automatically populate in the <head> section of your site.