First, you need to have a custom domain. You can purchase one at GoDaddy or any other domain name reseller. Once you have your domain name purchased, you need to point your custom domain to domains.tookapic.com using CNAME record. It's actually quite simple. All you need to do is add domains.tookapic.com as a CNAME alias. See example below:
It may look a bit different in your domain provider dashboard. But what you need to look for is CNAME records. If you have any trouble finding it, please let us know. When submitting a ticket, don't forget to mention the name of the company you registered your domain with.
If you got the CNAME thing done, you now need to go to tookapic website settings page and plug in your new custom domain.