When you are logged in to your iwantmyname account, you can set up Tumblr 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 Tumblr account. Their instructions to help are here Custom domains.
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.