You (or your domain name provider) will want to setup a DNS
CNAME resource record to point your domain name to
stores.ChannelAdvisor.com. The DNS record should be set up like the following:

www.yourdomain.com IN CNAME stores.ChannelAdvisor.com

Note, this will link www.yourdomain.com to your Basic Webstore, but the
domain without the www will not be linked. For cname records to work
correctly you have to have a prefix on your domain name, so you are unable to set one up for yourdomain.com.

However, to solve this you can simply put a standard redirect on yourdomain.com so that customer's are forwarded to www.yourdomain.com.

The next step in this process (if you haven't done this already) is to return to your store setup wizard (Webstore > Basic Webstore > Setup Wizard) and you will want to click the "Already have your own domain name?" link. Here you will just need to enter your domain name into the list.