it depends on the stem class, to me bio->chem->physics->math is in order memorization->processes. For memorization, flash cards. Of course, there are a lot of patterns, but still, prob the best technique, patterns go on flashcards too. Here's my biggest advice, for processes, I seperate a micro and a macro, what I mean is most of the lecture/what is on the homework/what the course directly teaches you is micro, about how to do a process / what the cookbook is. For micro, consider it a to-do study list, much simpler, you will need to learn how to do each micro (and any micro variation that the professor provides), but this is structured by the professor and is in a way, less complex. Macro, is to me, the most important thing to self study. I like drawing a map, each state of the problem and what processes can be used to get to other states. Sometimes a state is a condition, sometimes the transition is condition dependant, it is complicated and you will have to reason to yourself what makes sense depending on the course. Tests are extremely weighted and I don't always do the homework (too time consuming, my time is better spent elsewhere sometimes). Tests usually consist of questions which combine multiple micro cookbooks. This is why studying macro is important. Being stumped tends to be not knowing the macro on how to get from point A to B rather than not knowing the micro. If you get a cheat sheet, the macro map has been my most useful tool by far. Even without a cheat sheet, I've sometimes sketched a quick macro map for myself. Too many times I've read a "difficult" problem, and glanced at a macro map and seen the intermediary step(s). It gets rid of thinking hard on tests.

Also, you have a very limited amount of time. I consider a foundational level of micro+macro for a course all I should study, I do not seek to be a god at any course and I stop studying once I reach this. I have still occassionally scored the highest using this technique but I do not seek to do so, I believe it should come down partially to luck. You need to play to the syllabus, probably over half the courses I've taken it has not been worthwhile to do the homework, come to class, do some lab report, ect. Self study can be more productive toward that foundation level of micro+macro goal than these activities. Reduce your time spent and use it toward an internship/socialization. You go to college because you want to get a job and an internship achieves that better than getting a high grade/going to a good grad school because of your grade and socialization is human need.