No license required in the USA to buy, sell, make, own or use a silencer in the 38 states that allow unlicensed civilians to possess them.
Obtain the ATF form 1, form 5330.20, two fingerprint cards and two passport sized photos, fill them out, send them in along with a check for $200, wait 6-8 months for approval then start making parts after the canceled tax stamp comes back.
You will need a lathe to make a decent silencer, but it is not hard to do. You need to make it right the first time because you can't make replacement parts unless you pay another $200 tax or send it to an FFL/SOT class 2 for repairs. Even then the repairs can't change the bore or lengthen the tube.
Get forms here.
Order them here. You will need the NFA finger print card.