How should one prepare for the entrance exam for a postgraduate program at ISI Kolkata?

If you are going for a Master’s degree just for the sake of getting good pay, then don't, it'll be a waste of your 2 precious years. Let me give you an overview: 

• M.Tech will not fetch you a 6-digit salary in an instant unless you are placed in a top company like Google. 
• If you're interested in going for M.Tech in CS then plan ahead- see what is asked in the ISI exam. You will find that it's mostly the B.Sc level Math.

If you are in Kolkata, then definitely go to ISI. There's a bookstore behind the college, it has a Math book especially for the ISI entrance test. You may buy and prep from that.