How do I drastically change my study routine asap?

Hi everyone, I don't want to come off as narcissistic, but I have never needed to study to get good grades. I've gotten away with breezing through homework in minutes, often starting around 10 PM, and reviewing for tests just 30 minutes the day before.

Now I'm in my junior year, with three weeks left in the semester, and I'm facing two B's. It has finally sunk in that I need to drastically change how I study. I don't want all my work maintaining good grades for years to go to waste. And I may be in uni soon and I know I wont be able to breeze through like I used to.

I want to learn how to focus when I get home and actually study effectively during my study time over the next few weeks to lock in for my final tests and prepare better in the future. What are the best methods and routines for this kind of drastic change and developing better study habits in general for the future?