To get started, click on your avatar's picture in the upper right corner of the screen, then click on Avatar:
Next, click on Create Your Own Look next to the list of avatars, or click on the Custom tab at the top of the screen:
Next, you get to choose the Face, Hair, Outfit, and Accessories for your avatar.
Once you've made all your selections, click Next or Checkout, and you'll be shown the total Avatar Tokens it will cost for the changes.
You'll still have access to all your previous avatars if you'd prefer to not create your own avatar's look.