Use ShipMyGun they send you a label and you ship it off. You can check the list of FFLs on the ATF website to be sure you're shipping to an FFL. Include a copy of your drivers license so the FFL can properly log it into his book - and the name contact info of who you sold it to so the FFL can contact them to come pick up their gun. - Don't ship the ammo - it's a different set of rules. Easy to do but not worth it for a couple hundred rounds of 22LR.
They are part of Bud's and they become the shipping FFL so you are really just their agent dropping off for them at the shipper.
www.shipmygun.com