A few days ago, I went for a minor operation. When I was finally pushed into the theatre, the people worked there for a while. I can't even remember exactly what happened, but I got the feeling that they were starting the operation, and the thought 'Aren't I supposed to be asleep before they start?' crossed my mind, and then I became aware that I was back in the ward and I was done.
Now that got me thinking, how would one write an experience that the character does not quite comprehend or remember? A dream, events under sedation or drunkenness?
The easy answer is, you don't. You just leave the event from the text and refer to it later.
However, how can we learn if we always pick the easy path? Let's see how we should go about it.
First off, remember that this is an incomprehensible event. It does not follow the normal order of things. We see this error mostly in dream sequences (I've done it myself many times). Most dreams are actually for the most part crazy. The dreamer doesn't really know what's going on, he just thinks he does. Mostly in dreams, things don't make sense. So make sure it doesn't. Flashes to different events and people doing things they cannot do. Dreamers think it as normal, but the reader shouldn't. It might also be a good idea, in the event of muddling of mind, to show your character's confusion.
Next, the problem is that the reader should be able to figure out what's going on., to some extent, at least. This is sort of a counter for the first point, but one has to find the middle ground. If you make it too confusing, the reader will just get lost and glean nothing from it, if he or she even read it to the end. A good idea is to show certain points that can lead the reader to a conclusion, but make it obvious that the character has no idea what is happening (unless of course, he does).
Lastly, don't make it too long. Also, don't put it in if it serves no purpose. What good is a dream about a man walking alone in a hospital if it has nothing to do with your plot and do nothing to alter your characters? Nothing, that's what.
As a final note, I just want to add that dreams are often a way to pad your text. Only resort to dreams when it is necessary, not when you need a higher wordcount.