I suppose it depends on what your comfortable with. I mix all of my shades beforehand. I start with my darkest shadow tone and work that into the deepest shadow areas. I then take my midtone and work it up next to my shadow tone blending the two as I go. I use a stroking motion (just like painting). Some say to butt the two colrs up to each other and then use a brush (NO PAINT ON THE BRUSH!) in a "stippling" motion on the border between two colors and softly "stipple" the edges to blend them together. I suppose again, it has to be what you are comfortable with. Mike Davidson's book, "How to Paint WWII Figures" (hope that is right) is a great basic reference for getting started. It is readily available, and should be fairly cheap. I would mix a small amount of color up and practice on something small and disposible. Practice makes perfect! HTH,
Jay H.
OKC