Not very difficult, just take the rifle to the shipping company of your choice and have them ship it. The only requirement is that it goes to a FFL holder, your buyer will need to provide this. You may want to take a copy of the FFL with you when shipping to prove that's where/who you're shipping to.
In addition to the above post, you have to go to the hub of your shipping company. FedEx Office or The UPS Store will (most of the time) not ship firearms. Go to the UPS Customer Service Center or FedEx hub to ship. You can use USPS, too.
FYI if you use UPS you can insure it for 5000.00 dollars then it is practically hand carried through every building it sees , after it is tagged and bagged ,last package to go in the trailor and first one off by hand. But it needs a min. of 5 grand ins. for that type of handling.