I’ve written previously about how I create various layers to build up a painting to its finished state. Below is a different way of starting your underpainting and it’s one that you might find highly effective. I’ve kept the explanations really simple!
Step 1: Block in your image in pastel. This will be your initial sketch.
Step 2: Fix image with white acrylic paint. This will soften the colours you used in your sketch and create the basic shapes of the painting.
Step 3: Finish the image using acrylic as you usually would.