Hi,
I have no economics background, but have been selected for the MSQE programme in ISI. Here's the essence of my eco prep -
1. I began usually, using Varian as a textbook for micro; however, that was short lived as I quickly migrated to online courses and problem solving.
2. I googled anything I didn't understand.
3. I stumbled upon this forum late. But better late than never was never more true in my case.
4. Mathematics is really important to ISI. Have a firm grasp on real analysis and linear algebra (at least), they're both used in economics.
5. It paid to be open about my learning. No one book, or one singular resource or even a coaching class is going to be of any use. You need to be open to learn, relearn, ask, be wrong, unlearn and relearn again. I used Khan Academy, Coursera, random youtube lectures, and open course ware of universities.
As an example of this openness is the question linked
here. A question on binomial identities (in the syllabus) is solved using complex numbers (not in syllabus).
This helped me in this years PEB which had a question on the Solow growth model, that used more polynomial graphing techniques than economics.
P.S. - You posed this question only to the people who qualified ISI. That was exactly the kind of closed mindedness that you should ward yourself against. There are many people more brilliant than the ISI qualifiers who couldn't qualify the exam. It's the matter of one's performance on one day in a few hours.
All the best!