well first, go to control panel, you can find it under the large SmG icon at the top of the site
After this, you should see the control panel, or CP.
Scroll down a little bit, and you will see something that says your profile on the left side.
Click the one that says edit profile. if you went to the right area, you should see this.
scroll down, and you'll see places to put your usernames and gamertags. Then at the bottom of the screen, when finished, click "update profile" and you're done