Be sure to read the contest rules before you sign up, though. The rules have changed over the years, making certain types of costumes ineligible for entry. There are much tighter restrictions on how big your costume can be this year, and the BlizzCon organizers would really like it if you could walk up a few stairs in the dark without assistance.
After you sign up, you should receive an e-mail confirmation that your submission has been received. You won't know right away if you've secured yourself a slot, however, so just keep your fingers crossed and stay patient. And should you not get into the contest initially, there is still hope! In the months and weeks approaching the convention, some entrants may drop out of the contest and open up a slot for you. Good luck!
So, any WoW Insider readers planning to cosplay at BlizzCon 2013? What are you hoping to go as? If you need some inspiration, check out some of our costume galleries from previous BlizzCons.