Anki + practice.

1. Find a good textbook on the topic that is around the right level for you, and that also has many practice questions in it. 'Good' is determined by a combination of reviews plus an initial skim of the book.

2. Read and work through the textbook. Use Anki to memorize the key points/equations and do the practice questions as you work through each chapter.

3. Continue to use Anki for several months/years to review the things that you learned. This only takes a few minutes a day, but the benefits are astounding.