An underpainting is an initial layer of paint that blocks in shapes or values for subsequent layers. Applying an underpainting early on can help you determine that your composition works and allow you to see and plan for proper values.
1. First place the composition on the canvas using paint in a color similar to the planned color of the background. In this case, combine alizarin crimson, burnt sienna, and ultramarine blue to create the reddish-brown color for the outline.
2. Fill in the painting with a thin layer of underpainting to help you see the values. This allows you to confirm that you are still satisfied with the composition of the painting and see if anything needs to be changed. This thin layer of paint will dry quickly; you will be able to continue working soon. Note that these colors do not have to precisely match the final colors; you just want them to be close enough to get an idea of what the future painting will look like.
Images and text excerpted from The Art of Basic Oil Painting