There is no absolute best way. Something works for one person but may not work for another.
First of all, you should evaluate yourself what is your current knowledge level of English language.For this, you can first thoroughly go through the IELTS exam pattern. After you know it and cleared your doubts, just give one practice test on your own from a standard book. That way you can know your reading and listening skills level on IELTS band scale. For writing and speaking skills test, you can ask your friend who is good in English or a teacher to evaluate your performance.
According to your goal and current skill level, you should determine how much work you need to do in all 4 departments.
I would suggest to clear your grammar fundamentals and strengthen your vocabulary slowly and steadily using standard books like Word Power -Norman Lewis and others.
To improvise:Reading : Try to read English newspapers as much as possible until you get bored. You can read novels , I personally like Dan Brown’s novels and read them all.
Listening: Watching English movies and series is a great and fun way to improve your listening skills and also the pronunciations. You can stream English news like BBC for some time everyday. Even listening to English songs helps in a way.
Writing : I am not so good at writing but I think your writing will improve with practice and by reading articles of good writers. Vocabulary and grammar again plays a big role here. Practice is the best way for this. Wrote the same essay again for 4-5 times and find ways that you can improvise and avoid mistakes.
Speaking: My speaking skills are not so good, but average. The best way to improve is to speak as much as possible so that it comes naturally. Do not worry for the mistakes, it will automatically improve with time . With time, fluency and confidence will also increase. Speaking in front of the mirror helps too.
All in all, learning a language can’t be done in s short period of time. Exposure will gradually increase your skills.
After your preparation, give as many tests as possible to get yourself used to the technique and so that you can finish the test without any problems in time and so that you don’t get nervous on the test day.
Good luck! 🙂