Heres my version of the instructions for amending a return.

You may want to save a copy of the original for your records, I personally dont keep a computer copy of the original, its wrong, so I have no use for that anymore.  I keep a paper copy in the client file, or a PDF of the original.

Open the return as originally filed. 

Activate the 1040x by marking the correct year at the top of the form, switch over to the state (if applicable) and activate that amended form as well by entering the year in at the top.

Now go back to the federal return and make the needed changes and then go to the 1040x and write in the explanation of what's being changed.

Now switch over to the state return and open their amended form and you should have a checkbox to carry the same explanation over to the state, you may need to edit that for state purposes, depending on your explanation..

You should be good to go now.