Step 1: Modify A-Record and CNAME
Within your domain registrar's interface, find the "DNS Management", "Zone File", or similar area of their control panel. Here you will modify the A-Record and CNAME values.
Change the A-Record value to the IP address associated with the site. There are two places that you can find the IP address that your domain needs to be pointed at. One is the initial email that is sent when the site is added. Secondly, you can also view the IP address for an individual site under the "Site Settings" within the Site Builder.
Most registrars will already have the CNAME value for 'www' set to point to your A-Record. If there is no 'www' CNAME value, you can create this record and point it to yourdomain.com. This is to ensure that both www.yourdomain.com and yourdomain.com resolve to your DevHub site.
Step 2: Ensure your site is live
It can take 24-48 hours for the new A-Record values to propagate across the internet, but if the values were set correctly your domain should now resolve to your DevHub site.