Check out the documentations I made for a product: DOCS, page 18 explains where to place PAK files and page 19 is about having to package twice due to unreal engines internal structure.
A lot of other useful information in the document about all things related to GooglePAD (Not the implementation itself though, as that isn’t required for the product to work.)
Hope it helps!
EDIT: Updated the link. Seems to have died.