lol
Well with an existing jet in the skin there is a file called Decals.ini - open that up and you will see
[Decal001]
MeshName=vertical_stabilizer
DecalLevel=1
DecalFacing=LEFT
FilenameFormat=A7TailL
Position=-4.9,2.21
Rotation=0.0
Scale=4.5
DecalMaxLOD=4
[Decal002] etc
Should be obvious where the decal goes via the Mesh name - you will see that the rotation , position and scale can be changed very accurately. An easy way to get positions is to skin a plane that already has a complete decals ini.
If the original skinner hasnt used decals - then you may have to use trial and error - sometimes even changing the size or position can cause them to disapear or cause decal leak and other weird things.