@mstrauss Hey.
If you are going to host the blog and your website on the same server, all you need to to is to make sure that any subdomain is also pointing to your current server IP. You can do that with an A/CNAME record. I would just point a wildcard *.
Now just create another website with Forge on that same server. What happens behind the scenes that Forge will create a virtual host so when people access blog.sitename.com they will be served from another directory than when they access sitename.com.
If your blog is going to be hosted on a completely new server, then just ensure that in your DNS records you make an A record for the subdomain that points to the new server's IP address.