You'll need to send a letter to the Firearms Technology Branch declaring your intent. Usually, they'll request a drawing and/or photo. At that point, they'll send a letter declaring the weapon exempt.
The address is the same as the NFA office:
BATFE c/o FATB
244 Needy Road
Martinsburg, WV 25401
From what I understand, they're pretty easy to deal with on this.
Sometimes it’s nice not to destroy the world for a change.