Owner to FFL of buyer for long guns. Also, owner must send photocopy of DL with long gun. Receiving FFL must agree to receive long gun from private seller prior to shipping. Some will accept from private owner, some won't. Check before shipping to be sure.
I have bought from someone out of state and all he had to do was send a good quality copy of his state driver's license to my FFL holder,I believe he sent it as an email attachment.My FFL holder sent him a copy of his FFL and he then shipped the gun(not sure how he went about it,but he did) directly to him.I then picked up the gun and did the federal paperwork from my FFL.
It's my understanding that not all FFL holders will take shipments directly from a non-FFL holder so you need to find one who does.