Custom 3D Clothing with R15 Avatars

I’m trying to figure out how I would go about “welding” or “attaching” 3D Clothing (shirt) to R15 chars … but I can’t find any solutions anywhere & no clue where to get started .

(Trying to get the shirt / pants by themselves & that breaks the clothing’s bones ?)

How do people that upload 3D Clothing to the Catalog make it so it animates with R15 Characters ?

[Video Examples]
Shirt (Catalog):

Outfit from Catalog that I cut in half: