Step 1. Stamp the image with black ink
Step 2. Use a non-porous surface to create an art palette with your markers as shown here. I use the darker colours on the palette.
Step 3. Either using a blender pen or a lighter shade of pen colour you are wishing to use, pick up some of the colour from the palette onto your pen tip.
Step 4. Start colouring at the edge of the space you are wishing to colour (remembering to think about where your light source is coming from and where you would like areas to be shade more than others) colour in small circles as you go. Add more colour to areas which need to be darker and then slowly fade the colour by using a lighter shade of pen or by blending some of the darker colour with a blender pen
Here is the finished result
