Check out Youtube also, I have found great stuff there. Search for scale models & washes, and you should get some good results.
As Don said, it depends if you are going for a pin wash, for very specific areas, often recessed panel lines and the like. A lot of tutorials use oil paints diluted in mineral spirits, but others have said, other paints will work also. A sludge wash is typically spread out over a large area, and then excess wiped off, leaving some wash in recessed lines and around raised detail.
Personally, I have never had great luck making my own sludge washes. For sludge washing I have used Flory washes, but they are not cheap (at least for a tight-wad like me). I have to be very careful with Flory washes though. If I get it in to tight spots, I cannot get them washed off well, and I have had a terrible time in areas that were a little too rough or porous, as I could never get them clean. That said, I am working on a Bearcat that I painted dark blue, and the only thing that I could get to work at all was a Flory wash (probably more a comment on my skill level then any particular technique).
Do some searches here in Techniques and on youtube, there is some really great stuff around.
Experiment, and good luck!