The Japanese have a name for Little Red Riding Hood, but not for Goldilocks. Strange, that . . .
Anyway, mining for sentences is probably the simplest, easiest way to gain exposure supplemented with a dollop of attention.
You simply go through a book in the target language sentence by sentence. Consider each one.
If you can read it easily, do so and move on to the next. If it is difficult, then skip it and go on to the next.
Therefore, spend no more than 5 seconds on any sentence.
If there are no easy sentences, find an easier book.
If there are no difficult sentences, find a book that’s more advanced.
Very good. That’s the exposure part of it taken care of.
As for paying attention—‘study’ if you like that word better . . .
Whenever you come across a sentence that ‘catches’ a little, but only a little, then make some sort of mark. Underline the last word, or highlight the full stop. Don't waste time. Continue.
After your reading session, go back and write out in full all those Goldilocks sentences. That is enough to focus your attention for a few seconds at a time.
And that’s all that you do.
Eventually you may accumulate 10,000 sentences, as someone online once advised people to do.
But don’t try to study or memorize those sentences or enter them into a spaced repetition system. I don’t think that you need to. You’ll get more out of doing more of what you’ve done.
Of course, you may want to, but that’s a different matter.