There are a couple steps you need to take in order to get this done.
Step One: The A Record
The first step is done through your registrar: you need to point the domain to the Guild Launch servers. This is done with an A Record. The full details on how to do this depend on your registrar, but here are a few sample instructions:
Make sure to point the A Record at 126.96.36.199 - this will get it halfway there, to our servers. This change can take up to twenty-four (24) hours.
Some registrars allow both www.address.com and address.com to be pointed with one A Record, and some do not; this ultimately depends on the registrar you're using, but you may need to set up two A Records during this step.
Now you're ready for...
Step Two: Adding the Domain
Once you've pointed the A Record to our system, you can add it to your Guild Launch site! This is done on the Admin menu, under the 'Site Structure' tab, by clicking 'Site URLs.
At the bottom of that page is the option to 'Add A Full Domain to Your Site' - just enter the URL in the text field, click 'Add Domain' and you're set! The final result will end up in the 'Currently Active Domain(s)' category:
That's all there is to it! The domain is linked, and once the A Record is fully up and running on your registrar's end, users can use that domain to go to your site on Guild Launch.