How do I start preparing for IPM IIM Indore, effectively?

The first thing you should focus upon is brushing up your grammatical skills, alongside your vocabulary. You should understand the fundamental topics of the 10+2 mathematics section, including basic functions and probability. 

Most of the quantitative ability questions, while being essentially hard to solve, are of the level of class 9 and 10. However, there are times when certain questions can be trickier than others, even if you are good at Maths. 

Practice improving your calculation speed since the time allotted is crucially low. The time allotted is 75 minutes with 45 minutes reserved only for the quantitative section.