You submit all required paperwork along with your $200 to the BATF people. The approximate time frame used to be about 90 days, don't know what it is now. When you get their approval REGISTERING THE GUN, NOT THE BARREL, then you can obtain or cut your 14-inch barrel to install on it.
I sent mine to the old Scattergun Technologies (since bought out by Wilson) to have it done.
Vang Comp would probably do it for you, on receipt of the appropriate paperwork confirmation.
Either outfit does first rate work, and Vang can also backbore & port your barrel if you want to spend the money.
Either way, it'll need some sort of a front bead or sight, and a gunsmith can cut the muzzle squarer than most home tinkerers can. For strictly buckshot that may not be much of an issue, for slugs it can be.
Is it worth it? Depends totally on why you want such a gun. For a toy, no. For a serious defensive shotgun, yes.
Denis