Hello Vinayak,
Please find attached the updated OML.
1. I have added the logic on the ProductDetail Screen to add the record on the AddToCart button.
2. This record will appear on the ShoppingCart screen where you can go from the link View Cart available on the FourColumnGallery screen.
3. In the ShoppingCart screen you can add/remove the quantity of the products and the sub total will get updated based on the add/remove the quantity.
Please review the OML and let me know, if you need any other help.
Thanks & Kind Regards,
Sachin