Good question! I probably spend the most time playing and reviewing the games afterwards. Those reviews usually involve looking up specific positions in books or professional games. So, it involves study as well. But I find that allowing my games to dictate my focus keeps me directed towards the things that are relevant to my level. I also tend to remember the details better because they are connected to actual experiences. I also try to spend time just reading and watching videos (usually Sibicky and sometimes Dwyrin), but I tend to find them most useful when going back to them when reviewing a game.
The other big thing has been soliciting comments from stronger players. L19 has been fantastic
But I also have found it very useful to take regular lessons. (I realize that this is a controversial subject.) I've been taking lessons from Alexandre Dinerchtein for a while now. I don't mean for this to be a plug for him (although, if you are looking for someone, he has my endorsement!), but instead, I'm advocating that one find someone who could be a mentor or like a coach. Part of it is, of course, expertise. But I think the most useful part is having someone with that expertise look at your games regularly. Consistent attention allows them to better direct your study by seeing patterns across your games. Things that may not be clear in a single game review.
Finally, one thing that never seems to get discussed in these "how to improve" discussions is the specific skill sets of individual players. I suspect that different types of study would be useful to different players. For example, my visual memory (the ability to look at a picture and then recall the details of the picture later) is one of my weaker skills. So, I can't just look at a particular position and remember it later without special effort. But my ability to remember relationships between things is VERY good. So, once I work with a joseki for a while and begin to understand the role of each move, I retain the information much more easily. I think this is also one of the reasons that I also am more likely to remember critical points and move sequences when I see them as part of a game review. Within the context of the game, the role of the individual moves is often much clearer.
I've also noticed that I tend to get more out of books in which the authors organize the information along more general principles than books in which the authors are more focused on the details of specific positions. Examples are useful on occasion, but a series of examples with little explanation of how they relate to each other is not. I work from general relationships to remember specific details, rather than vice-versa. For example, I bought Cho Chikun's All About Life and Death. It's a very thorough book -- it provides the status and correct playing sequences for myriad positions on the side and corner. But he provides almost no guidance on how the different positions are related. He shows you the details of how to handle individual positions, but the most he does in terms of drawing connections between the different shapes is to assess slight variations. I read through the first volume, but retained almost nothing. I ended up using it largely as a reference until a few weeks ago when I started trying to make my own connections between the various shapes. That's provided the framework that I need to remember the details in the book and has made the information much more valuable.