I highly recommend making sentence cards in the SRS of your choice. The gist is to read, watch, listen to, etc., some Japanese, and then, when your brain picks out a word you want to learn, make a card with a sentence containing that word. The front of the card is the sentence in Japanese. The back of the card is the sentence in Japanese and its translation in English (for bilingual sentence cards) or the translation in Japanese (monolingual sentence cards, for when your grammar and vocabulary have advanced enough to learn with such).
The benefit of sentence cards over a single vocabulary word per card is getting to see the word you want to learn used in context. The sentence you use for the vocabulary word does not have to be the sentence you found the word in. You can extract example sentences by searching through online sources such as dictionaries. However, using the sentence you found the word in is of course okay. Another benefit of this approach over simple word -> definition cards, in my own opinion, is that sentence cards are far less boring than vocab cards. Another benefit on top of this is that you can weave grammar examples into your vocabulary learning as well.
As with most methodologies, creating sentence cards requires balance in order to not become a chore. I can’t take credit for the following tips, but I can vouch for them.
Firstly, when creating sentence cards, follow what some call “i + 1.” “i” is taken to be your current reading/listening capability, and the “+ 1” means that what you are consuming is just a little above your full capability. So, you’ll be able to understand some percent of what you’re consuming, say 50%, and the rest of what you read/hear will be unknown vocabulary and grammar. “i + 1” is important because if you’re consuming content far beyond your level of competency, you’ll be looking things up so often that you run the risk of associating consumption with being a chore, and this is demotivating. Additionally, if you create sentence cards that are way beyond your competency, you won’t have any idea what any content on your cards mean, and this also runs the risk of hampering your learning efficiency and motivation.
There is also the question of how frequently one should create sentence cards. If you are, say, reading a book and end up creating a sentence card for every other sentence you read, you’re probably going to, again, burn yourself out and associate reading with being a chore. It’s okay to only create the cards every so often and to allow yourself to read something, think “I don’t know this word,” maybe look it up really quickly, and then continue reading instead of creating a card. Prioritize fun. It is highly unlikely this is the last time you’ll ever see that word, and so you’ll have another opportunity for learning as you read on.
Finally, when you master the meaning of a sentence on a card, burn/retire/delete the card. Your sentence deck will grow alongside you in sophistication, and we can retire cards that become child’s play to us.
Your deck of cards will seem a bit silly as you just start out because your reviews will be for, say, 6 sentences you end up learning by heart. But by adding sentences each day, you’ll quickly grow a collection that will be very diverse.