What utility do I need to create a skin for a plane?
There should be some pointers here for you
But does this apply to FSX?
Aircraft pain is done in photoshop or maybe gimpshop in dds format (i beleave thats dx texture design) I beleave you would simply make a new folder in C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes called texture.5 or whatever number its up too. I would suggest using the existing textures to create an entirely new one or replace an old.
Available textures are defined in the aircraft.cfg i beleave and named accordingly. Pretty easy stuff right there.