- Click this week's hero
- Click create banner
- Next
- Next
- Enter nickname and Battle.net email and you're done!
Unfortunately I didn't find this until recently so the first three heroes are gone. There are still two left. HOWEVER, the FAQ says there will be an opportunity to unlock all the sigils after the remaining heroes are released (probably in the final week).
The final day to claim is May 18th. It's a limited release freebie and you won't be able to get it after the promotion is over.
How do the class sections and sigil lockouts work? What if I miss a week?
Throughout the promotion, each class section of the website will progressively unlock every week and stay unlocked. In-game class sigils, on the other hand, have limited availability and do not stay unlocked. The week that a class unlocks, only that class’s sigil will become available via Mark of Valor, until the next class section unlocks and its respective sigil takes its place. If you missed a week, don't worry, there will be another chance to earn your favorite class sigil later in the promotion, but please note that sigils are only available for a limited time. May 18 will be the last day that you can earn an in-game sigil.
Read the whole description and still have no idea what a sigil is.