How can I legally attach a muzzle break on a barrel to make it the full 16 inches?  Does it have to be pinned and welded, or would just welding do the trick?  I am in NY if that makes any difference.

Thank you very much.

Most people do pin and weld over the pin. Some also use the high heat silver solder method but I hear that can discolor your barrel. JB weld and Loctite are NOT to be used. I only mentioned the last 2 as some have mentioned the use of those.

are there any instructions for doing this, or do I just find a gunsmith who will do it?

I just removed pinned and welded brake from an Armalite.  The pin was installed the same as the FSB pins, except is looks like they drilled a blind (not all the way through) hole.  The pin was installed and then the one hole was spot welded over.  The pin went through the side of the original threads.  The pin was installed from the bottom of the brake going up so the weld was on the bottom of the brake and less visible.

If I was going to do a pin job, I would mill or dremel a small flat spot so the drill wouldn't walk off the side of the brake.  On a mill, a 1/8 center cutting end mill would probably work to start the hole.