Instructions
Go to any Scout Base on any of the islands and go to the Monster Synthesis Booth near the back. Speak with the girl and she will give you options for synthesizing your monsters.
Click "Synthesise" by highlighting it with the directional pad and pressing "A."
Select the first "Monster" option at the top to pull up a list of your available monsters. Place a Great Dragon in this slot. Scroll down the the second "Monster" option and place a Bone Baron in this slot.
Press "A" to bring up a prompt screen. Press "A" again to confirm that you wish to initiate the synthesis process.