To host your site under a domain you own, you must
- first upgrade your account in Websites for Trello; after that,
- create a card in your _preferences list and name it
- in the description of the card, write your domain name, and only that;
- then you must point the CNAME record for your domain to
<your-subdomain>.websitesfortrello.com. If you never did this before, it can be confusing, but in fact it is quite easy. Here you can find some guides on how to do it specifically for your registrar (the website where you bought your domain). If your registrar isn't there, searching for its name + "cname record" will probably give you many hints on how to do it.