That process is pretty simple.
If the clothing have its own armature then you unpack the clothing (just unpack, not FULLY) and you drag and drop every bone it has into your model's armature where it will be affected. Arms go to arms, legs go to legs, etc.
You just keep doing that. You don't need to do head or neck if the clothing won't be affected by those parts.
If its just the clothes, then you unpack it like I said before and you drag that onto the model. Done. Be sure to resize it if needed of course.