When you are logged in to your iwantmyname account, you can set up Big Cartel with your domain really easily from our one-click app Marketplace:
Just follow the steps and in a matter of seconds your domain will be properly configured.
Then you will also need to add your domain to your Big Cartel account. Their instructions to help are here Set up an iwantmyname custom domain.
Make sure that in your Big Cartel account setup you include www at the beginning of your domain, like www.mydomain.com, rather than just the root domain: mydomain.com.
You may see things start working within a few minutes, but please remember that it can take up to 48 hours for DNS changes to fully propagate around the internet. So your website may not show up in all browsers immediately.