Unless there is a state or local ban or licensing, you need to find the suppressor or contact a Class 3 dealer who has or can get you one. Then you apply to the (Federal) Bureau of Alcohol, Tobacco and Firearms on a Form 4 for transfer of the suppressor, and pay the $200 transfer tax. Your dealer can help you with the forms. (You have to have arranged the purchase first, since the serial number has to be on the form.)
You also have to get the signature of the chief law-enforcement officer for your area, usually a chief of police or sheriff.
The wait for approval may range from 60 to 90 days, sometimes more.
I suggest you locate a Class 3 dealer in your area to find out about local laws and about the attitude of the CLEO. Some refuse to sign any Form 4, others just sign as a matter of routine.
P.S. Don't buy a suppressor unless the owner is a Class 3 dealer or can show proof of registration with the Feds. Possession of an unregistered one can bring stiff penalties and make you a felon who can never legally own a gun.
Jim