As Willie Lowman said, complete a Form 1 and send it in.
Also you should be able to speak with the company you are interested in buying the 12ga conversion insert from, and they will probably be able to walk you through the paperwork to keep you legal. Unsure which company you are looking at, but I have noticed a couple require a copy of the approved Form 1 before they will process your order.
Hope that is a help
Last edited by Fishing_Cabin; August 7, 2012 at 12:17 PM.