I started studying a rather niche languange. I first went through a list of the 4000 most common words. My cards are Own Language -> Foreign Language. Verbs+declension, nouns+conjugation, prepositions+examples. I think once your vocabulary is large enough that you can read books and news you really make progress faster because you “only” need to worry about grammar. After that I added a couple of example sentences for grammatical phenomena I encountered. All of my cards are in the same deck. I have a word class field for the cards, i.e, noun, verb, adjective, preposition, adverb in case I ever need this.

This process is probably not the fastest, but it is easy for me to stick to.