Setup the Subdomain
Open your site manager
Go to the Firebase Console, select your project. Then click on Hosting in the left sidebar.
If you have not already added your second site, go to the previous step.
Select the site you want to add as your subomain. Make sure you aren't adding the subdomain to the main site.
Add custom domain
Select "Add custom domain".
Enter the subdomain you want to use, including the domain name.
Adding redirects
You can choose to redirect here as well.
So for example, if I wanted to redirect read-it-somewhere.bertramdesigns.com to rits.bertramdesigns.com, I would check the box to Redirect.
You will still need to add these DNS records and the ones for the main subdomain (rits) to your domain provider.
This will supply you with the DNS records you need to add to your domain provider.
If you need pause for any reason (like transferring the domain), you can "close for now" and it will give you a "needs setup" tag for you to fix later.
Add the DNS records to your domain provider
I use Hostinger so I would go to Hostinger, select the domain, and add the records there.
The quick setup suggests adding a CNAME record for the subdomain. There are some indications that this may not work if you are have other Type-A records. Instead you can use the Advanced setup if there are issues. More information here
Verify the domain
Depending on the TTL of your DNS records and your domain provider, this could take a few minutes to a few hours.
Once the domain is verified, you should be able to see "connected" next to it.
Thats it! Time to deploy your site to the server. Next step