in my experience, it usually takes a strong empathy link, and skill astral traveling, as well as sleeping at the same time as target. to try. you may wanna start learning how to astral travel in your sleep, cause that makes it easier, (to do that, just start the meditation for astral traveling and gently fall asleep), after you are comfortable with that, try to tap into the link you have with the target, and follow it and astral travel through. don't think of their physical location though, that can screw it up. as for form changing, that is hard to do, cause you have almost no control of the dream world, you usually pick a form and you might be stuck with it, but depending on the strength of their mind you may be able to change, but i honestly never tried, i just use the same form.
First focus on learning to lucid dream. You need to know you are dreaming to take control. Once you can lucid dream, you need to practice playing with the details of your dream. I used to do this as a kid, changing my dreams as I went along for fun. When you can do that, you will become aware of out of body experiences while sleeping. You can't change the environment when traveling out, you can only control yourself. Yes, you can fly or shape shift if you practice. Have fun. Now start trying to leave your body consciously while in a dream, since you know what it feels like.
During the day when you are not sleeping, you should be researching psychic self defense (dion fortune wrote a good book) and practice it. You should also be setting up wards in your bedroom and under your bed because the more you progress, the more attention you will attract...not all good attention.
By this point you should know how to defend yourself magically, have protections up, and be able to control yourself 100%