Verify its an FFL you are shipping to, they dont have to legally send you their license but at least get their number and check it
HERE.
Actually shipping it is like anything else, box it up and go to Fedex, declare it as a firearm and ask for overnight service. Include your paperwork etc for the destination FFL in the box.
Not all FFLs will take from an individual though, you may need to have a backup plan of having an FFL to send outbound through....