It's not that easy, and (in most of cases, like 99%) requires 3D models of windows - as they weren't meant to be decalable.
But if you want it, here's fast list:
- Get model of window,
- duplicate it, and move it just a little, little bit,
- assign any transparent texture to duplicate's material,
- remap model properly (just in case)
- export models, script textures.
- Then use any graphics program which supports alpha channels and put decals in proper place.
