Howdy.Well this is how I learned and it seems to work O.K. though I'm going to try it in ink. I'm now using oil paint, the type in the tube and mix black and burnt umber. I'd say about 80% thinner to paint. You want a tinted thinner and not a thinned paint. After the future has cured a couple days I take a small brush, maybe a #1or #0 and run the mix into the lines, recesses and around the details I want accentuated.Try to get where you want but don't worry too much about overrun.
After about 20 min I'll take a small piece of paper towel folded over, or you could use a Q-tip moistened in thinner, not soaked, moistened and clean up any spills or overrun.After it dries a couple hours I may go over the process again if I think it needs it. Once it's how I want it I let it dry a day or two and dry brush with a suitable lighter color to bring out the detail.
Some guys use acrylics and like Isaid I'm going to try it with some inks. No matter what you use it's pretty much the same.As with any technique if you can practice it on a junker or spare parts it may give you an idea of what you're doing. Here's a link, the example is aircraft but it's basically the same. Have fun!!!
http://www.swannysmodels.com/Weathering.html