Looked it up on the ATF website about a year ago for a machinist friend.
There is a form to print off and other instruction, go straight to the horses mouth is my opinion.
You will fill out two copies of the ATF Form 1 5320.1. They will both need to be originals and signed by your CLEO. Enclose two recent 2x2" pictures and two completed fingerprint cards. You can order the cards from the ATF website here: Distribution Center Order Form | ATF
You want the FD-258LE cards.
One copy of the 5330.20 Compliance Form.
One check for $200 paid to Department of the Justice or the BAFTE
You will need to have an idea of the OAL and caliber you will make your suppressor as you will have to put that on the Form 1. Oh, and pick a serial #.