Instructions
Power on the Xbox 360 console, select "My Xbox" and log in to your Xbox Live account.
Select your current gamertag/Xbox Live avatar, then select "Change Gamertag."
Select "Enter New Gamertag." Enter "Shiska" or other desired gamertag. If the desired name is unavailable, you have the option to "Get Suggestions" where a handful of similar available user names are listed.
Select "Yes, use this gamertag" to save the changes.