How to make wash paint for miniatures?

I used only Citadel paints to do my Warhammer figures for years. To make a wash, simply thin down paint to an inky consistency, I always used plain old water to thin. Only wash over well dried paint, or it will come off. Usually just a drop of wash on the brush is enough to fill in cracks and deep details. Start with a little bit, and add as needed, you can add more but you cant really take it away if its too much. Citadel used to make washes in many colors, they had blue lids, but it's been several years since I've bought any. I painted miniatures for years and always got compliments for my work. the real trick is layering. Base coat, paint, wash, more paint to highlight, then dry brushing for extra depth.