1) Make sure the person you're going to dream-walk is asleep. You can do this usually by time. It's possible to put them asleep, but that's another technique.
2) Get into a relaxed position, one that if you fell asleep in, you would wake up stiff in. I find the one that works best is laying down on my back.
3) Form a clear image of the person. Imagin how they look, how they sound.If you're intimate with them, imagine how they feel, how they taste, a favorite moment you had with them, or a very rememberable one.
4) Will yourself to their dream. I find saying their name helps.
5) Observe. Just watch what's going on.
6) If you're going to change anything, just make it change. The worst thing that could happen is they just change it back. Though usually, if you're fast enough, once you start controling their dreams, they won't take it back.
7) Just will yourself back to yourself. You could just wait around untill they wake up. You'd be pushed back into your body, anyway.
Magi A. Silverwolf
Now that the air's clear, let me get to the point of this essay/how to.
Stories of dream-walkers (people who read dreams) have been found in nearly any culture you would find a dragon... nearly all of them in one way or another. In ancient times, they were considered mystics, shamen, or voices of the various gods and goddesses. In Christanity, they are witches or prophets. In modern times, they are usually considered luntics or phonies.
Some times, they keep their power silent, using it only to help a loved one. Some times, they lose the control that's so fragile when dealing with dreams and so very necessary. Some times, they get lost on their way back and their bodies slip into a coma. Some times they can be with their beloved when their bodies are far apart. I could go on, but I think you get the idea.
From the seed of hypnopathy, vines of both good and ill can sprout. I need that to be clear before I go any farther. This is not just fun; this is not just a game. What harm is done by dreams can only be undone by dreams. Hypnopathy is not a good idea for revenge. It will echo back into you. And I can tell you from experience, once the seed of a nightmare is planted, it's very hard... it's like a weed.