Mead93’s answer really covers the answer to what you asked, but it seems to me that perhaps the “when and what” are not actually your question. It seems to me that what you really want to know is, “How do I even start?”
Reasearch and study generally proceed from the general, more global levels of information down to ever more specific and detailed levels. So, general study of some topic, say a particular battle or campaign, might create in interest in something related to it, say a particular type of tank. Further study and research might ultimately land you at a level where you want to know the specific dimensions of a particular nut or bolt on that tank so that you can accurately replicate that same nut or bolt in scale on your tank model.
Study and research can also be cyclic, so the effort can also lead from the very specific and detailed up to the more general and global. In other words, the process described above can also go in reverse. So…
You might also revisit that earlier generalized study and research, which triggered your interest in that particular tank, in order to continue with the model build. Perhaps as you build the model you also want to add historical, geographic and environmental context to your final vision for the display of that tank model (how was it marked, how should it be weathered, what sort of terrain was it used in, what was the weather like, etc.). You might then go back to that initial reference source to find (or re-read) that information needed to add that context to the model that has been improved or enhanced with the later, detailed technical research.
The real “trick” (if that’s the way to put it) is to approach deliberate study and research like all “problem-solving” efforts, and ALL problem-solving paradigms (i.e. process models) BEGIN with identifying and defining the problem. That is, you MUST know what the question is BEFORE you can find the answer. What is it that you want to know?
(Random, creative processes are different, and sometimes random “study” can lead to inspiration and motivation to undertake a more deliberate effort necessary to engage in a creative, artistic activity. That is, perhaps you’re reading a book about some campaign or battle simply for pleasure and enjoyment. As you read that book you find your interest peaked by something, and that interest develops into some specific idea or vision for a model or diorama. Now that you have that vision, you also now have a specific question or direction for deliberate research. So, do not confuse such random “study” undertaken in the search for ideas and your artistic muse as “deliberate” research. The one may lead to the other, but they are different mental processes.)
Once you KNOW what your question is, then the collection the information needed to answer that question can be done. Here is where your initial query of “when and what” should be recorded comes into play. The answer to that is simple (yet not necessarily easy). It is to record by taking notes of any information that you do not think you will be able to recall from memory when you need it as you implement the answer to your basic question into whatever creative effort you’re undertaking.
Your note taking may be as simple as jotting down the page number that some useful bit information is on. You know or feel that in a couple of months’ time when you’re ready to start planning the composition of the diorama or vignette base for your model that you’ll want to go back and re-read the details of the battle in which your model tank was involved and which also sparked your interest in that tank to start with. There may be other details mentioned in that reference book that you also want to recall or refer to later, so you may also write down those page numbers.
In some other reference book, say one about that particular tank model, you discover the production timing that helps to determine just which specific physical details would be visible on the tank model you want to build. You might then continue adding page numbers to your notes, perhaps referencing particularly useful photos or drawings.
As you continue, you might find that you want to add some detail to the model that is not included in the kit and which cannot be found offered by a credible AM product vendor. So, your notes might start to include your own hand-drawn sketches of this detail, maybe with dimensions that you derive by scaling various photos found in several different reference books. You might also record the page numbers and titles to find those photos later as you scratch-build this detail.
Think of the final product of this research as your own, personal bibliography, index and footnotes for the project that you’re working on. You want to have recorded the information and sources that you will need to refer to as you actually undertake that project.
In short, there is no one research recording process that can possibly cover how you save information for future reference as you actually undertake the implementation of the answer to your initial question. You just have to record and save what you, yourself, know that you will need later, and do so in a manner that suits your own needs and purposes.
Your own needs and purposes drive the techniques and formats for making your notes and recording your research findings. YOU make YOUR recording techniques and formats to fit YOUR needs and the way YOU work or want to work.
Finally, you should understand that even though you have identified your initial starting question (so that you can know what it is that you WANT to know to answer it), the process itself of learning and study to answer that question inevitably leads to more questions and branches and sequels to your original research.
You should also know and understand the researcher’s corollary to Murphy’s Laws: No matter how long and diligently you study and search for information, the last bit of missing information you needed for your project will always manifest itself as soon as you’re finished with that project. LOL!
The practical importance of this “Researcher’s Corollary” is that you have to eventually decide that you have all of the information that you need to move forward even if you don’t have every bit of information that you might want. Once you reach the point where you have enough to do a credible job (“credible” being defined by YOUR own standards), then you should make the call to get started. If you wait until you have every bit of info you do want, it’s quite likely that you’ll never start, much less finish you build. Sometimes “good enough” is as good as it gets.
Don’t be discouraged! Even the experts don’t know all the answers to every question nor all the information applicable to the subject.
