Open up the 1040X worksheet and check the year, put in the explanation, etc and then go and do the same for the state if you have one.  Once the amended forms are activated, you can go in and make your changes and the 1040X should automatically update for the changes.  In the official Intuit playbook, they say to make a copy of the client file before you amend.  I keep paper files of the original and amended forms so I skip that step.