Multisite - Serve static content from a cookieless domain

I would like to "Serve static content from a cookieless domain" on my multisite at I need help setting it up. I have found a lot of information on the internet so am more than a little confused. At this point I have created a new sub-domain called I have configured A record.

My wp-config.php file already has
define( 'WP_CONTENT_URL', '' );
define( 'WP_CONTENT_DIR', '/home/mary/public_html/mary-content' );

Instructions are to add
define("WP_CONTENT_URL", "");
define("COOKIE_DOMAIN", "");

but I can tell by looking at it that that won't work.

I have also seen additions to make to .htaccess file but again, I don't think they will work.

Can you tell me how to proceed please. I need to know
1. what do I put in wp-config.php
2. what do I put in .htaccess
3. what do I do next? i.e. can I use wp-rocket, or do I need to load all CSS files for all sites in the static sub-domain?

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