Because the ATF needs to provide approval to add NFA-regulated items into a Gun Trust, there is a three-step process that must be completed in a singular order.
The Grantor (the person preparing the Gun Trust) will transfer NFA-regulated items to the Gun Trust using an Assignment Form that is provided as part of the documents.
But this Assignment Form cannot be used until the ATF has reviewed the Gun Trust, approved the registration, and issued the Tax stamp to the applicant.
So the order of process is: submit the Gun Trust to the ATF; await approval and the issuance of a Tax Stamp; create an Assignment form to move the item into the Gun Trust.
Additional instructions can be found as a part of the Gun Trust documents.