It's actually very simple.
Code:
AddEffect = Attachable
   PresetName = Coattail
   Mass = 2
   Sharpness = 0.1
   HitsMOs = 0
   GetsHitByMOs = 1
   SpriteFile = ContentFile
      FilePath = *YourMod*.rte/*TheIsolatedCoatSprite*.bmp
   FrameCount = 1
   SpriteOffset = Vector
      X = -*Half of the width of the sprite (In Pixels)*
      Y = -*Half of the height of the sprite (In Pixels)*
   AngularVel = 60
   AtomGroup = AtomGroup
      AutoGenerate = 1
      Material = Material
         CopyOf = Grass
      Resolution = 4
      Depth = 0
   DeepCheck = 1
   JointStrength = 5000
   JointStiffness = 0.6
   JointOffset = Vector
      X = 0
      Y = 0
Joint offset is where on the coattail it connects to the actor.
Then, inside the actual actor part of the code, put:
Code:
   AddAttachable = Attachable
      CopyOf = Coattail
      ParentOffset = Vector
         X = 2
         Y = 7
This is where on the actor the coattail appears.
That basically makes the it appear on the actor 2 pixels to the left 
(when the actor is facing right), and 7 pixels down.
Replace all the things in the asterisks (Leave the "-" on the sprite offset part).
Also, copy/pasting it will work fine. But you have to change the stuff in the asterisks.