How to make the best use of study aids
Law professors often advise students that all they need to do in order to do well on the exam is pay attention in class and read the cases. What they really mean is that all the concepts they will test you for on the exam will at least be mentioned either in a case or in class discussion. But, that is not quite the same thing as saying that reading the cases and paying attention in class is sufficient to get you a good grade. As a law student, you are extremely pressed for time and you are being inundated with new concepts at a lightening pace. While it probably is true that if you had 5 years to absorb the material, you could get everything you need to write a perfect exam answer from class and cases, in a more realistic law school experience, you are usually going to be focused on trying to get a basic understanding of the material quickly enough. Study aids can help immensely in that regard.
I think of study aids as falling in three categories:
Simplified doctrine
Judges and law professors love to revel in nuance and complexity. Most often, professors pick complex cases and emphasize the subtlest points in them during class discussion. They do that partly just because they personally find that stuff more fun to think about. It is great to be able to summon some of that complexity in an exam. But, before that level of nuance can really be applied, you need to have the basic legal structure nailed down. You want to understand and address all the elements of each tort, for example, before you go into the application of the peculiar, but interesting, analysis used in that oddball case the professor spent an hour on. Some study aids are designed to give you that basic foundation much more clearly, and in a form much more readily applied in exams, than cases do. Examples of this type of study aid are the Nutshells and the Examples and Explanations series.
Detailed doctrine
Some study aids go the opposite direction. They cram every wrinkle of the doctrine into exhaustive tomes. This type of study aid is typically called a supplement, hornbook or treatise. They range from say 800 pages to 50 volume sets. If the professor does recommend using supplementary materials, this is the one they recommend. These supplements can indeed be useful for filling in gaps in your understanding.
Testing your knowledge
The third category of study aids is those that test your mastery of the subject. The Examples and Explanations series give a bunch of short answer essay questions and the Questions and Answers series gives a ton of multiple choice questions. These study aids typically focus each question on one individual concept that you need to get straight, so the idea is that by the time you work through all the questions, you've forced yourself to learn all the material.
How to leverage study aids
You can find the full list of supplements I recommend for each course here and a list specifically tailored for 1Ls here.
As for how to use them, the real answer is that every approach that keeps you constantly struggling to develop your understanding of the topic works. Which exact path works best varies from person to person and course to course, and to some extent, you just need to figure it out for yourself. Most importantly, do not try skimping on reading the cases or going to class in favor of study aids. The aids are a useful addition to the core material, not a substitute for it. It is generally a better idea to try out all the different tools at first and then cut out the ones you find less useful than to start out just trying the one you suspect will work best.
That having been said, I can tell you what worked best for me. In the courses where I got the best grades, what I did was to read the Nutshell throughout the semester, reading each chapter roughly around the time we covered that material in class. I found that doing that helped me frame the issues raised by the cases. Often times, the professor would gloss over concepts that to him are quite basic, but which were totally new to me. The Nutshells are great for filling that information in and helping you store away the details you learn from the case and class in the right drawer in your mental filing cabinet. Then, either before I did my outline or while I was outlining, I would read the Examples and Explanations. The summaries of the law are generally less detailed than those in the Nutshell, but they serve as a great refresher, and the questions started me down the path of testing myself. Then, when I was feeling particularly ambitious, I would mix in the Questions & Answers series of multiple choice questions. The great thing about those is that it can be useful even if you only have 10 minutes at a time to dedicate to it. And, they are a bit less demanding than the other ways you study, so they're good for when you're tired or less focused. They can also be kind of fun in a weird way.
I generally did not make use of the treatises. During my whole time at law school, I probably only used them a handful of times to fill in things I was not clear about, and each of those times, the fact that I had to resort to the treatise to answer the question meant that it would never come up on an exam. That said, some people take a totally different approach where they study only the treatise and cases. That wouldn't have worked for me, but maybe it is worth checking one out at the library and seeing if you're one of the people who it does work well for.
Lastly, and I hope this goes without saying, you need to take a boatload of actual, full length, practice exams. That is perhaps the most important step in the whole process, so don't skip it. I would recommend completing at least four full length, timed, exams for each course, and I actually found it useful to skim through a lot more than that as well just to see if I was missing anything.
Some study aids go the opposite direction. They cram every wrinkle of the doctrine into exhaustive tomes. This type of study aid is typically called a supplement, hornbook or treatise. They range from say 800 pages to 50 volume sets. If the professor does recommend using supplementary materials, this is the one they recommend. These supplements can indeed be useful for filling in gaps in your understanding.