Shoot a ray from the camera to an object. Then 'connect' it to all light sources, by measuring the change in angle, and properties of the surface. And add up.

Shoot a ray from the camera to an object. Then 'connect' it to all light sources, by measuring the change in angle, and properties of the surface. And add up. If the surface is refractive/reflective, recursively shoot one more ray calculating the right direction, with correctly diminished intensity and follow the same process.