Casting Shadows?

How do you make an object or avatar cast shadows?

You don't.

SL doesn't cast real shadows.

You can fake it with textures for objects that are stationary, but that is it.

Cast shadows would be the biggest single enhancement to the SL visual experience (which is already good).

The textures used on the prims can have shadows built into the texture. This means it's a fixed light angle, which has no bearing on where the sun is, but it does look good. Most of the building textures that I make and sell use this trick for greater realism. Clothing designers use this as well, adding shading where they expect it will be on the 'underside' of a surface, such as the lower half of the breasts on women's clothes. It takes some artistic talent to pull off, but looks great.

You can also place a partially transparent texture just above the ground, under an object, to simulate a shadow. You could, for example, make a texture that looks like the shadow of a tree, and position it just above the ground at the base of a tree, and it will generally look as if that tree is casting a shadow. But the shadow won't change as the sun moves.

But as far as real shadows go, Avatars and prims in SL don't stop light at all. 'Sunlight' and other light sources still illuminate a completely enclosed space, even though the light source is outside the enclosure. A light source only illuminates the faces of a prim that are "facing" the light source. So at least the outside of your house doesn't glow when a lamp near a wall is lit. But that same lamp still does cast a glow on the ground outside, as if the wall was transparent.

By the same token, all light sources in SL are omnidirectional. There is no way to cast a focused 'beam' of light, other than making that 'beam' out of phantom prims that glow somewhat and are mostly transparent.