Hi Kasun,
I just passed the CISSP exam two weeks ago, here's how I prepared:
I studied by myself, you will need to buy a study guide, please find the link: https://www.amazon.sg/Certified-Information-Security-Professional-Official/dp/1394258410
I listen to/watch this video or podcasts on YouTube, if you need any particular source please let me know.
And finally, I have done 1000+ questions on the exam library, in both study and simulation mode.
The preparation took me about 4 months and about 200+ hours, before I went to the exam, my passing score from the exam simulations was between 72-83% correction rate.
Hope this is helpful to you.