If you want to send to an individual FFL holder you can search for registered FFL dealers in your area. Popular websites like gunbrokers usually have search feature you can use. Or you can ask for a referral. Also gun shops are already going to be FFL dealers, so you can use them as well for the transfer.
Everyone will have a slightly different fee for the transfer but $25-30 is typical. This is for the overhead of accepting the shipment and processing paperwork.
A private party usually needs to contact the FFL holder and exchange some info, including license, serial numbers, and shipping info. Once that is done the transfer can occur after you've paid the private party. I recommend using a payment method that you can dispute the transaction if the private party should fail to deliver. Try to avoid cash or money order deals.
The shipment goes to the FFL dealer. Prior to exchange you will have to fill out standard background check paperwork that the FFL dealer submits. Once cleared you can take possession of the firearm just like you would if you were buying it from the store.