Work in progress
Go to https://secure.m2osw.com/user and log in.
Then click on My Domains (a tab in your account, click on the image to the right to see details.)
Here you will see the number of domains you can use at this time.
To add a domain click on the Add new domain button at the top. With each domain you are also given a certain number of sub-domains. You add those later. Once you added a main domain, you get a new link "Add a sub-domain" on the line of that domain.
At this time, you can add one domain name per Snap! website you own.