Instructions
Navigate to Xbox.com in your web browser and click "Sign In" in the upper-right corner of the page. Enter your Xbox Live credentials and click "Sign In."
Highlight "My Xbox" and click "Avatar Editor" from the drop-down menu.
Select a male or female avatar on-screen to use as a base for your new avatar. You will be able to change all of your avatar's features and clothes, so don't worry if your avatar does not look much like the avatar you want to create.
Use the "+" and "-" buttons on each side of your avatar to to adjust its height and weight.
Click "My Features" to edit your avatar's skin color and facial features. Click the box that corresponds to each feature you want to change. You will see your changes appear on your avatar in the right-hand side of the window.
Click "My Style" to edit your avatar's clothes and accessories. Click the box that corresponds to each item you want your avatar to wear.
Click the "Save" button underneath your avatar to save the changes to your new avatar.