In what proportion should the given ingredients be combined to form a cookie?
It is nearly impossible to reverse engineer an industrial cookie recipe from its ingredient list directly--especially without the final product to compare to.

Some of the ingredients may be unique to the industrial process (such as "glucose" or "emulsifiers"), hard to obtain, or proprietary.  Some of the steps may require special processing or equipment.

Furthermore, your local labeling laws may be different than other places, so we would not know how to interpret this list of ingredients.  For example, in the US, the ingredients would have to be listed in order, by weight.  

Instead, you should search for recipes that produce the kind of cookies you want that are designed for the home kitchen.

I cannot speak to local products in India, but here in the US there are many web sites or blogs where authors try to recreate recipes of popular commercial food products.  These recipes are designed for the home kitchen, and may not have the exact same ingredients or techniques as the commercial product on which they are modeled, but are instead designed to have similar outcomes.  I don't know if such sites exist in India, but I would suggest searching to see what you can find; failing that, I would search based on the name of the cookie for recipes for this type of cookie.



In terms of some of the specific questions:


What sugar should be used? White or Brown? If this was the US, it would always be white unless otherwise specified, but your local labeling laws may differ.
Can vanaspati be replaced with anything (clarified butter)?  Probably, but that will affect the taste and texture of the cookie.  Many industrial recipes use ingredients which are less expensive than the ideal ingredient, to keep costs down.
Custard refers to custard powder or the custard which is ready to eat? That would depend on your local labeling laws; here in the US custard would never be listed as a single ingredient without parenthetically describing each of it's ingredients as well.  I would guess powder if I had to guess... but listing it like this may be part of how they keep their actual recipe secret while complying with local labeling requirements.
Why is glucose needed when the sugar is already there? It is almost certainly present to alter the texture of the cookie in some way, either interfering with crystal development of the other sugars at some part of the process, or for its hydroscopic properties to help retain moisture.  This is the kind of ingredient that is often used in industrial recipes, but far less commonly used in home recipes.
What should be added as Emulsifier Agents?  Again, it depends on what your local labeling laws permit, and these may be ingredients that are rarely used in home cooking.  The most common emulsifier in home baking is egg yolks, which contain significant quantities of lecithin.
Water isn't mentioned here but should I assume that it is needed? Maybe, but there was also that "custard" listed, and we don't know what it means or contains.