Entering someone elses dreams has absolutely nothing to do with astral projection. It is known as "dream walking". People do it at times without even meaning to. I personally believe it is possible, due to the fact we are all connected on an energetic level. Even with years of practice, it is not always something you can control.
If you really wish to do this, research "dramertime". It may tske a bit of digging, but is a worthwile project. Things you can do to help with your efforts would be dream journaling, learning to sense energy shifts when someone goes to sleep, learning to recognize when you are dreaming (lucid dreaming), gaining control over your dreams, and I would suggest finding a practice buddy once you get to acutally dream walking. This can be a lengthy process.
For those whom are wanting to do this to mess with people or control them.. dream walking does not give you control over the person. It is also not the best idea to use to mess with others. Our minds do have nstural protection when in a dream state. When in someone elses dream, their subconscious is in control. To try to alter things will most likely cause them to go on defense mode and either end up with you tossed out or in a nasty nightmare. I am not accusing the poster of having such an intent, but others have joked about giving others nightmares and I figured it was worthwhile to mention. Enjoy your research!