Well, most paints are either enamel or acrylic. The acrylics (like GW) uses are all water based, so thinning them down is easy.
Enamels, on the other hand, are oil based. They need special enamel thinners, which most paint manufacturers offer with their enamel paints. They are sometimes used as brush cleaners, but also for airbrush work. I use them every so often to reclaim a paint pot that has dried out.
For either one, the method of thinning paints down is fairly simple:
Get an eydropper and a box of strong toothpicks for stirring. Use the dropper to put in a small amount of your thinner (since you're talking about acrylics, this will be ordinary tap water) and stir it in until the thickness is to your liking. If you feel it is not thinning down enough, add some more water, but be careful to not add too much.
Any extra water will gradually soak into the paint.
Always be careful not to mix the water in by shaking the pot. For whatever reason, acrylics bubble when water is mixed in that way - I found that out on one of the TT special features, actually.
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I forgot to mention that, with enamel paints, you have to be careful to not put in too much thinner, because that can ruin the enamel. There's no such worry with acrylics, or at least I've never seen it.