Basically it's used like lightmapping, here's how it works:
type into your cfg:
texture 0 textures/default.png (defines the parent texture)
The "texlayer 1" sets the texture that will be blendmapped. To initiate blendmapping, apply the new texture to a surface, then press 2 on your number pad (make sure you have numlock off), then just click away!
0 turns off blendmapping, the mouse scroller changes the kind of brush you're using for blendmapping.
Hope this helps!
До свидания и заткнись