Select the domain you wish to purchase, in the case multiple results are presented, and click 'Continue'
The next screen will ask you if you would like to purchase added features along with your domain. As you can see in the graphic, you can get email or hosting packages. If you are going to host your website at WA, then do not waste your money on these add-ons. All those features are included with your WA subscription.
All you need to do is click the 'Sign Up' button under the Instant Domain option.
You will typically be offered to purchase additional domains with your chosen domain name and alternate TLDs. It is not necessary to buy all these if you don't have a need, .COM, .NET, or .ORG TLDs are all you should be messing with at this point. So go ahead and skip past the additional domains and the next offer of the 'Complete Protection' security package.
You will end up looking at your Cart so you can see your total purchase cost:
Make sure everything is ok and click 'Continue'
Going in and changing the DNS settings is also easy, but now it tells me I have to wait up to 24hours for the settings to take effect in WA so I am currently here twiddling my thumbs waiting for that to happen so I can start building my webpage :)