I consider myself a decent hidden player, despite never really trying to get HD scores ever. Though, the majority of my DT scores are with hidden.
1) I have never practiced HD by itself, ever. I've only "practiced" it when I felt like FC-ing something with it, which is very rare. I've found that just playing and getting better at regular maps and even HR mod, helped me with HD, because instead of paying attention to approach circles, I pay more attention to when the hitcricles themselves appear, not where the approach circle is.
2) I only remember where notes are if there are very awkward unreadable stacks or rhythm. This doesn't happen too often though. I am generally able to figure out the rhythm of most of my DT HD plays without having played the map ever. Most stacks are relatively easily readable for me because you can see stacks. If you can't see the stacked notes, then you just have to memorize them. However, sometimes you can guess where the next stacked note is, because it feels like the music is still going but you don't see a note, so there must be a stack either at the beginning of the slider or at the end that you can't see.
3) Reading streams is a combination of when you see the last note of the stream and listening to the music. Most streams end on the main beat (unless the music otherwise dictates), and depending on when you see the last note of the stream, you can generally judge/feel when to stop. Also generally you start a stream with the same finger you end with (unless the music has dotted rhythm, which are much less common to see when streaming).
After a while you will just get a feel of when notes will appear and disappear. Alternatively, you could just play hidden all day I guess. Honestly, I wouldn't worry about hidden too much.