Using a flashcard system like Anki can help maintain long-term knowledge. To use flashcards effectively, it's important to analyze how they fit into the larger task of reading and review.

Understanding and Prioritization: A Chicken and Egg Problem

We can't make flashcards of every bit of knowledge conveyed in our textbook. How do we know what's important and what's not, unless we are already familiar with the subject? After all, the sentences in the textbook can be:

• Core knowledge that we want to make into flashcards
• Underlying principles or equations that are extremely important, but will come up so many times that we don't need flashcards to remember them.
• Details that we can re-derive or infer if we remember the "core knowledge"
• Tangents and examples that help explain or give context to the core knowledge, but aren't critical to remember
• Filler material

Many learners attempt to make notes and flashcards as they go, but lack the understanding that would let them prioritize better. Others just skim or read, then return later, but will have forgotten much of the material along the way.

Mini-Towers

As I wrote in The Multi-Tower Study Strategy, some topics are split into discrete, minimally-intersecting "boxed topics." This makes them easier to learn, since on your first encounter, forgetting about one won't damage your ability to understand the next. 

Other topics are "tower topics," which build on a set of fundamentals, layer by layer. You are constantly at risk of forgetting the fundamentals, even while new information is piled on. This makes them harder than "boxed topics."

Of course, almost all topics are "tower topics" on some level. A single page of dense biochemistry might introduce a multi-step reaction, requiring you to absorb and understand a number of chemical names, reaction types, products, and the way the overall reaction fits into larger pathways.

If you read closely, sentence by sentence, looking up terms you don't know, you can individually remember and understand via your short-term memory. But as you plow through more and more information, you can easily get overloaded, forgetting the beginning by the time you reach the end, even though you understood each step.

Simply going back and re-reading immediately is a form of "massed practice," and may be an inefficient use of your time.

Most learning comes in the form of "mini-towers," and learning how to deal with them requires a more sophisticated approach.

Progressive Highlighting

Progressive highlighting is a fairly simple procedure, particularly if working from an e-book or PDF viewer that permits reversible highlighting, or using a pencil or sticky notes in a physical book. It works like this:

1. Decide the approximate number of flashcards you want to make per day, or per page of the textbook. For example, you might choose to make 2-4 flashcards/page on average.
2. As you read, reversibly highlight any material that you think might make a good flashcard. This lets you make use of your "in-the-moment" knowledge while you read, without committing to a hard prioritization, and with no constant switching between reading and note-taking/flashcard-making.
3. Timebox your reading. I suggest about 6-8 minutes per page of dense STEM material. Understand it as best you can in that time, but do not worry about trying to attain perfect understanding. If a page is easy to understand, you can "bank" the time and use it for harder pages later on.
4. Once you've completed a day's worth of reading, return to the start. Remove any highlights that no longer seem like useful flashcards, until you have the planned 2-4 flashcards/page.
5. Turn these remaining highlights into flashcards. Gwern reports an estimate of 30-40 seconds required to review over the first 3 years per flashcard.
6. In parallel with flashcard practice, schedule an occasional re-read of the chapter. One re-read might take place the day after you finish the chapter. Another might take place a week after that, and another at progressively lengthening time intervals. These re-reads will be much faster and more solid due to the flashcard practice, and will help build your overall mental model to contextualize the flashcards.

If you're reading three 40-pg textbook chapters/week, producing 120 flashcards for each chapter, and doing this 52 weeks per year, it amounts to 3 chapters/week * 52 weeks/year * 120 flashcards/chapter * 40 seconds/flashcard * 1/3600 hours/second = 208 hours/year of review, or about 35 minutes/day of flashcard practice.

In addition, you'd be looking at an average of 2 hours/day of reading.

This amounts to a total of about 18 hours/week of study, about 1/3 to 1/2 of the total recommended study time for a full-time student taking three 6-credit classes. The rest of the time can be used for problem sets, homework, labs and exams, as well as for re-reading previous chapters to synthesize your flashcard practice.

Recap

One approach (good) students take to study, which I think is common, is read + flashcard making -> study:

1. Read the textbook. As you go, take notes or make flashcards of anything that seems important. Take as much time as you need to understand the contents.
2. Study with those flashcards, or use notes as a more convenient way to look things up so that you can avoid filler in the textbook.

The "progressive highlighting" approach is read + highlighting -> make flashcards -> study:

1. Read the textbook. As you go, highlight anything that seems like it would make a good flashcard/note. Timebox your reading to balance understanding and forward motion.
2. After finishing the day's reading, remove any highlights that no longer seem like useful flashcards. Turn the rest into new flashcards.
3. Study with those flashcards. Occasionally do a faster re-read of the textbook with the benefit of your practiced flashcards, to help synthesize the material.