You should go to the NFA/Class 3 forum in the Armory and read, every question you have will be answered.
In sum, get one of the gun trusts online that are NFA specific or from an attorney that knows NFA and the trust requirements. Get it notarized, you will have to have a beneficiary, that is a trust requirement. Then, just either go to a dealer or buy online and it will be shipped to a dealer you choose. It takes longer to buy from an online dealer if out of state because that dealer would have to file a Form 3 to transfer it to your dealer. If you buy locally (or from any dealer within the state and pick it up directly from that dealer), they just prepare the forms and you mail them in with your trust and a check for $200 for each item and then forget about the items till your Form 4 clears and the dealer receives you stamp. Then go pick it up, get the regular background check, and off you go. You will have to pay for the items up front. A Form 4 transfers ownership of a suppressor or SBR to you.
If you build your own suppressor or SBR, then you can efile a Form1 and add your trust in and they will email you your stamp and then you can build it.
It is pretty straight forward once you do it. Definitely, read on the Class 3 forum. You do not record a trust in Missouri. This is the basic process. There are a number of firearm/NFA attorneys in the state you check with for actual legal help. Do it right, otherwise, it takes longer.