To do a SBR, you fill out a Form 1 in duplicate. You will also need 2 FBI fingerprint cards and a Citizenship form. You can get the forms online at TitleII.com, but the fingerprint cards you will need to order from The ATF unless the CLEO uses the same cards. You will also need to attach a passport style photo to each form 1.
You use a Form 1 to "make" a SBR. Most of the time, this means slapping a short barreled upper on an existing Title I firearm (like an AR lower).
Fill out all the forms, and then go to the CLEO to get your prints done and have him sign both Form 1s. Ask him/her to use blue ink... It's easier for the NFA examiner to know it's an original signature.
Send the completed forms and fingerprint cards to the NFA along with a check or money order for $200. You should note the serial number on the check.
Then you wait. When the forms come back approved, you can "Make" the SBR.
For a Silencer, you will need to find a dealer that either has the one you want or who can order the one you want. Once teh dealer gets it, he will start a Form 4 transfer to you. Again, these forms are in duplicate, and need CLEO signatures, photos and prints along with the citizenship form and a $200 check. Another wait, then you're good to go.