Configuring Register.com to use your custom hostname
Step 1: Sign up for an account and register a domain at Register.com, if you haven't already. Log in and proceed to the Domain Manager page. Click Domain Alias in the upper left corner of the page.

Step 2: If you have any existing aliases for your domain that you do not use, please delete them now. They can conflict with the alias you are about to configure. When you have deleted them, click "Add Alias."

Step 3: Type out the full domain you want to use, such as www.yourdomain.com. Then make it point to domains.smugmug.com. Click the yes button to add the alias. 
Step 4: Double check that it looks correct on the Alias summary screen. 
Step 5: Type the entire domain name you're forwarding into the space provided by SmugMug in your control panel. 
Gotcha: They must be all lower-case or it won't work.
Don't add http:// or any / marks. Just type www.yourdomain.com (or photos.yourdomain.com, for example, if you set it up that way).
Step 6: Be patient. It takes 24-48 hours for DNS changes to take effect.
Let us know if you have troubles.