I am using Kanshudo for sentence mining as I am learning grammar. Each grammar point usually has sentences with that grammar. There will often be a “More examples” link that will give you even more sentences. I go through the sentences list and if I see ones where I can understand most of the vocab in the sentence, I will “favorite” it and the sentence will go into my Favorites flashcard deck. Then I let the SRS do its magic on the flashcards. It seems to work pretty well for me so far.
You can also make grammar flashcards with just the rules on them, but that doesn’t seem to be very useful for me.