Hidden is only a visual mod, so the map itself doesn't really get harder to play.
A good idea is to play maps you've already done well on/FCd with nomod, with hidden. This way, it's easier to pick up on the different visual cues in hidden, as you know how something should be played, and can pair that up directly with what you see on the screen.
After you're used to how hidden plays, it's more or less the same as playing nomod, except hidden notes.
For those, you'll either have to memorize, or predict/guess as you play. You get better at this as you play more maps and get better at the game.
Just as an example of prediction: let's say there is a note stacked under a slider during the first kiai time of a map. You hit this one, as you can see it due to stack leniency. When the same section comes during the second kiai, you can't see the note at all because of the shape of the slider. You can still predict a note being there, if there's nothing else appearing that would follow the same rhythm (i.e 1/2), and you know the rhythm for this section from earlier.
Also, skin and hitsound might help you at playing hidden, but they're generally not necessary to play well with hidden at all.
