You turn off automatic renewal after you activate the promotion. So if you didn't get the promotion yet, and you can still find it on the dashboard:
1. add a credit card to your account.
2. select the promotion on your dashboard and follow the steps to get the free month.
3. use the My Account link at the top right of this web site
4. Towards the top, in the big grey box, you will see the words Automatic Renewal, it will say On, click Change to turn it off, that way you don't get charged after your free month is over.
However, if you no longer see the promotion on your dashboard, then it's possible the promotion is over, which I have seen happen a lot in my time while working for Xbox Support. Hopefully you can still get it, too bad if not.