If you're going to manipulate someone else's dreams, it can be tricky because it involves using spellwork or energy work on the person's subconscious mind because that's the part that is most active during rapid eye movement (R.E.M.). The subconscious mind can be a very confusing and misleading consciousness and it would take a lot of practice to efficiently manipulate another person's dreams.
Entering another's dreams is sometimes called dream walking. I've never done this but I have talked to people who have and I've been told it's another practice that requires a lot of effot and trial and error.
Is there any particular reason why you would want to enter/manipulate a person's dreams? With more detail I could probably shed some more light on the topic.