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.