The process is similar for both an SBR and a suppressor. Identical in fact, if you are buying the SBR ready made. If you are building it yourself it a different form and there are some practical things you have to do such as engraving the receiver.
IMHO, start with a .22LR can (it will get more use, and will be most impressive in the level of suppression):
This photo is of a CZ Kadet owned by a local ARFCOM member -- we did a group purchase of .22LR cans from a local manufacturer.
Do some research on exactly what can you want to buy, find a local distributor if possible, if not, find a class III FFL -- look around for one who doesn't charge an arm and a leg for the transfer fee. The sad truth about .22LR suppressors is that the NFA Tax, and FFL fees can add up to as much, if not more, than the cost of the suppressor itself.
The FFL or dealer will giude you through filling in the form -- but check the M16-Full Auto group, they have some detailed descriptions of how to fill in the various forms.