A couple of questions if I may?

Hi gang. I've never attempted to weather any of the planes I've built, but I always wanted to. Nothing dramatic, just something to make the plane look something other than my usual "factory fresh". I bought a set of chalk pastels and I've been reading here that you "grind up a little pastel". What does that mean and how does one do it? I also intend to darken my panel lines, so I got some Grumbacher acrylic burnt umber paste which I intend to use for that. The instructions I read said to mix it with water to the consitency of coffee. I'm not sure whether that means the colour or the viscosity? Can somebody help with a little advice?
Thanks,
Rich