depends on your location.
i HIGHLY recomend consulting an nfa attourney in regards to trusts. its a little more expensive than using online law document makers but each state requires specific wording to make it legal in that state. nfa trusts make it easier to obtain an nfa item bypassing the permission portion of the local leos in most cases. trusts also make it easier to pass it to beneficiaries for inheretence.
once you have a valid trust for your state then you go and pick whatever suppressor you want. you can buy them online same rules apply to suppressors and nfa items as buying a handgun except the transfer must be to a class3 dealer or special occupation tax holder. here is a good site for nfa dealers http://nfadealers.com/index.php
obtain valid nfa trust or secure permission from local leo chief.
find dealer to buy from or transfer to.
find suppressor or other nfa item.
fill out nfa permission papers and submit with 200$ check or money order to federal govt i use money orders but not postal money orders.
wait 6-9 months then fill out transfer form just as if buying a regular gun and take your new toy home.