What is the best way for solving I E irodov book

This book is considered as one of the best book for competition. First thing you should do is identify the syllabus of the exam you are going for, for example if you are going to give only JEE then there are almost 30% questions which are not for that, and for Olympiad level the thing is different.

As in this book the questions are very conceptual, so you cannot directly start it. You must have enough knowledge of the topics before solving it. Like for Mechanics portion you should know the concept of mathematic tools, kinematics, NLM & friction because question is designed by concluding all these. Same you can see in Optics & Electrostatic parts.

The questions are of high level so to solve them you need to be conceptually strong and you need Time. So if you have enough time or say you have extra time then start it accordingly.

In case you have 2 years for exams & you are going to give JEE exam you have enough time, so after understanding the concept of the topics you can go for it,  if you do it on regular basis then there are 2-3 question per day you have to solve, so you can do it that way.

The key points are you must have time, you need to understand the concepts thoroughly, do it on regular basis.