leather belts

we are a manufacturer of textiles (only trousers). we receive po from one of leading company for manufacture and supply of trousers along with leather belt. the belt should fixed with each one trouser separately.
they will also sell it in the showroom like single product and GST will be charged @ 5 % . the leather belts are in 18% tax bracket.

now the question is how much GST should be charged 5% or 18% or separately and on what basis we have to charge.
please help me on this