Hi Ginny,
I would start by mixing up a true gray scale using only white and black. The main reason is to understand where you lightest light and your darkest dark falls on the value scale. A full range value scale is about ten or eleven steps including pure white and pure black on either end. In most instances the lightest light will not be pure white, and the darkest dark will not be pure black.
Try using just enough thinner to get the paint to move easily but not become watery and runny. Shoot for a consistency that covers the canvas easily with a stroke but doesn’t let canvas show through. Paint the lighter shades thicker and more opaque.
Try and mix the grays that are transitions between a dark and light form instead of blending the dark and light together by smudging the paint. This will keep you from over blending. When painting in grays probably not as important, but when you paint in color, over blending can dull the colors.
I hope you find this useful.
Randy