Want to know about Modular Kitchen

here is my plan.
-measuring the space for the kitchen
-designing the kitchen using softwares.
-calculating the amount
-----I have to buy doors and plywoods from another company.
----baskets, hinges, and other hardwars I have to buy from another shops.
-Then calculating total cost + miscellaneous expences + my profit.
and preparing a quotation.
how to calculate GST.