They are partly being of energy, after the infusing is done some cells are destroyed, dark energy is used to fill in that space, this comes however with the disadvantage of that they are weak to both things a normal biological creature would be and a energy being.

As for the process itself the subject is put into a tube that is similar to the ones used for genetic engineering, Dark energy is them carefully guided through the body by a machine that places it in various parts of the body, including the brain.