We do not know the details. That is why it is a hypothesis instead of a scientific theory. The general outline is that the basic chemicals that were present in the solar system when it was formed polymerized and those polymers were able to catalyze basic metabolic reactions (decomposition of glucose in response to energy from the sun, etc). The complexity increase due to concentration of the chemicals (probably either in crystals or at deep sea vents) and life originated as a consequence of the energy flows in the chemicals of the environment.<quoted text>
How does it happen?
We have details of early stages of this process and of later stages, but not many of the intermediate stages. On the other hand, there have been no fundamental issues that require supernatural forces at work instead of basic natural forces as existed on the early earth.
Do we have all the answers? No, of course not. But is it reasonable to assume a complex collection of chemical reactions that we call life originated from less complex collections of chemical reactions that we would not consider to be alive? Yes. part of the problem is deciding where, exactly, the line is to be drawn between life and non-life. At the boundary, it isn't so clear what the distinction is.