If I add a collapsible stock to my carbon 9mm pistol, what are the steps to make it legal???

You will need to either make shure the barrel is at least 16" long or register it as a SBR and go through the NFA process.

Quoted: You will need to either make shure the barrel is at least 16" long or register it as a SBR and go through the NFA process.

Ditto.

File the form 1 and obtain approval BEFORE actually putting the stock on.

That is, if you ever want to be able to legally own guns again.

Quoted: You will need to either make shure the barrel is at least 16" long or register it as a SBR and go through the NFA process.

the barrel lenght is 7.25", what form 1 and other process I have to go through?

Quoted: the barrel lenght is 7.25", what form 1 and other process I have to go through?

Go to www.atf.gov and download NFA form 1.  You will have to fill that out.  You will find more information there on how to fill out a form 1 and the requirements.  You will also have to check on State and local laws as well before doing that.

There is one other option.  You can just add the folding stock to your AR pistol without registering it and spend the next 10 years in Club Fed playing hide the salami with Bubba.