Most owners of CS:S will have seen the dust particles in the tunnels of de_dust. Well this tutorial will show you how to create this very easy effect.
To start off you need a location for the dust particles to be floating, such as in front of a light source. In this case I am going to use a simple hole in a ceiling.
We are now going to make the floating dust effect which is a brush based entity, so select the brush tool
and make a brush with the nodraw texture covering the area you want the dust to float.
Select the brush and press CTRL+T. From the menu, choose "func_dustmotes". you can change how the particles look by changing the "Keyvalues", but the main things to change are "Particle Colour" and "Particle Per Second".
That's it done. Compile and test.
Related:
Example Map
Cr4ig