1. It’s not officially supported by T-Mobile, so don’t bother calling support, they’ll tell you to go buy one of their handsets.
2. Go to the online T-Mobile Configurator and follow the steps to configure details.
3. Once you’ve had the details sent to your phone, make sure you activate them or nothing will change!

Why T-Mobile support couldn’t have told me to try their impressive online tool is beyond me, but they didn’t even mention it. Oh well, thank goodness for the Internets!