Right at the beginning of the mission head toward Nova Braga Airport. Better avoid entering it from the east side since this area is very well secured and will take a lot of your time. Instead try to pass the northern part and look for the northwest gate which is presented on the screenshot above. When the enemies from the other side will walk away you may enter.
During the mission you need to remember about few obstacles. The entire area is protected by the patrols of enemy infantry and Walker Gears. In various part of the airport you may find cameras which you have to avoid and you can't destroy them since it will also be alarming for the enemies. In few points are hidden mines and every explosion may also raise the enemy awareness.
Your destination is the building from the screenshot above. Inside this building you will find the arms dealer and the CFA official will also get there (although you shall try to get here before his arrival). Get to this building from north since it will secure you from being detected by two snipers from the roof. If the helicopter haven't arrived and you have some spare time you may neutralize these snipers and an "ordinary" guard who also patrols the roof.