During the freshman year of high school, I had decided to master pre-calculus on my own during a summer recess. So I spend 2-3 months reading each chapter and solving all odd problems (to be able to check against the answers at the back of the textbook). While I've learned the material, in retrospect, I think it wasn't the best time-efficient approach. Fast forward 15 years, past my university years and years on a job, I seemingly cannot master anything in depth; I sort of only scantly familiarize myself with a material. I only read the material or watch on-line video courses (such as Khan academy lectures, free video courses offered by leading universities, etc). I feel that I don't even know how to study or concentrate anymore. Always multitasking. Please advise on the development of effective study habits. How do you study independently these days?
Friends I am looking for the actionable steps, and not just philosophy. What and How I can arrange my study to effectively master a material.