There are several official ATF forms that are relevant to trust owners. Each one contains the instructions for completing it. Follow the links below to download each form that you need.
The Form 1 is the tax stamp application used when you want to create your own NFA firearm, such as a homebuilt suppressor or a short-barreled rifle.
The Form 4 is the tax stamp application used when you want to buy an existing NFA firearm, and will often be filled out by the dealer that you're buying from.
The Responsible Person Questionnaire is the form that must be filled out by every current trustee (including yourself) when you apply for a tax stamp.
The 5320.20 is a form used to notify the ATF when you wish to move certain NFA firearms across state lines, whether temporarily for a hunting trip or permanently if you move to a new state.