1) Go to http://wotcode.com/mmosite_christmas/
2) Click Create Account
3) Enter your email, desired in-game name, and password into respective fields
4) Click "Have an invite code?" and enter your invite code into the field
5) Click "Play for Free"
6) Now you should receive a confirming email from World of Tanks Team. Click the link in the message to confirm your registration with World of Tanks
7) Install and run the game. Enjoy!
*You can use your invite code only once while creating your World of Tanks account. Not for sale.