This method works on any New/Old account that is currently on silver status.
You need to have a valid payment option attached to your account. And you also need Xbox 360 console.
Sign into your account you want free 1 month on & Sign out once the dashboard have fully loaded. Make sure you're connected to the internet
Go to Home tab and click Explore Xbox Live
Now sign into the account you want free 1 month on
Click on Xbox Live Gold What's in Gold
Hit A for 1 month free gold
Confirm your Purchase
Note: If you do NOT see Explore Xbox Live tile, change your locale to US.
Don't forget to Turn off Auto-Renewal