Which study method to choose is a good question! It depends a lot on what you like doing, and on you want to do, or have to do. When I was at school and university, I had to sit numerous tests and exams, some of them quite important. In order to do that, I read and reread my notes and books, beforehand, and memorised a huge amount of information, whether that was explanations, facts, or grammar. No doubt, I have forgotten a lot of this now! For languages, I memorised vocabulary, verb declinations, and noun case endings. I also had plenty of practice translating or doing comprehension. I remember too listening to cassette tapes of pronunciation! But because I was studying almost every day, and doing lots of revision, I could indeed remember enough when it came to the tests and exams.
Now, I have no desire to sit any more exams. When I want to study, I like to read interesting books on a subject. This works well for history and literature. For studying the languages which I have learned, such as German and Latin, I sometimes give myself a refresher course by going back and looking over my old school books. Because I learnt from these, staring again at the pages helps to bring back the information.
I also like to translate, and then I choose something interesting, such as a short novel or story and work through it. I sometimes do this with a friend; if you have arranged with a friend that you will both look at a passage and work through the translation together, then you have an extra incentive to do this. This works even better if it is regular.
Personally, I find it much easier to learn from paper materials like books and notes, than from what I see on a computer screen. However, this is not the same for everyone. I like to write things down by hand sometimes, as this helps me remember. I have also repeated things that I want to remember, out loud or silently.
For speaking or listening skills, then constant practice will make a big difference. You could try describing your day to yourself in another language.
It may help to set specific targets for what you want to achieve. For example, I would like to revise the past tense by the end of this week; or, I will have written a paragraph in English in three days time. It does not matter if the end result is not perfect.
Finally, whatever study method I use, I try to get up, at least every hour, and move around!



Vocabulary:
declinations: to decline a noun, pronoun, adjective or verb means in grammar to give a list of its different forms, by grammatical case, and by singular and plural.

By Thegreenj - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2210607