First of all try some [easy] and [normal] diffs. They are pretty lazy, but it helps alot. So your brain gets practised to realize different colours (dons and kats). After playing easier maps for a few weeks, you can try harder diffs with some streams, and you'll see you can realize these kkkd dddk dk kd's much better. Now to your question : If you want to play with hidden later, you should look at the middle of the screen. When you want to have a good accuracy, look at the circle where you have to hit the notes.
You need just practise, practise, practise, and you'll get better and better. Just play some easier maps first to train your accuracy and stream-realizing-skills.
basically don't look at the left section because when they come it's gonna be too late, just look to the right where they come from. also i hope you're using windowed mode because i think fullscreen makes reading harder. and finally it's just a matter of practice, the more you play maps with fast dons/kats the better you will be able to recognize them.