They can't. The solar bodies are created only through alchemy. However, many people can easily have astral experiences because they 1) either created the solar astral body in the past and are now fallen, or 2) they achieved a certain level of spiritual development in the past (without entering into the Major Mysteries) and subsequently lost their powers through abuse.
Regardless of the fact that astral projection may be easy for some people, so long as they do not become serious and work upon their defects, those powers will be taken away and such souls will suffer incredibly.
For unto every one that hath (spiritual development through conscious works) shall be given, and he shall have abundance: but from him that hath not (since he does not work on himself) shall be taken away even that which he hath.