Heres the process.
1. Buy the supressor and have it shipped to a C3 dealer that will accept the transfer. Or buy one he has in the store, you'll need a S/N to start the paperwork. You must buy it first, but you don't get to take it home....YET!
2. Either the dealer will give you the correct paperwork or you can get it directly from the ATF's website.
3. Fill out the application for the transfer.
4. Get the CLEO sign off or have a trust. Not 100% yet on how a trust works yet. But at Least a CLEO sign off.
5. Get a Money order for 200 USD
6. Get 2 passport quality photos
7. Take 2 through 6 back to the dealer so he can verify everything is correct.
8. Send send 2 through 7 to the ATF. If you do it yourself at least let the dealer look over everything, any little thing can hang up an otherwise "approved" transfer.
Now its a waiting game you get approved when you get your tax stamp with photo and form 4 back from the ATF. And it really is a stamp, and mine was affixed with clear tape you would use for wrapping B-day gifts, so was the passport photo. No I am not kidding.
Take your form 4 back to the store and take home your supressor. The dealers I worked with in TX and LA both had the same policy, if you were denied they returned all monies except 10% that way you could feel a little better about risking so much money. But a good rule of thumb is if you can buy a handgun you an "probably" buy a suppressor.
IANAL
If I missed anything its due to my memory it has been a few years.