Using Black Paper

Don't worry. It's not going to be a gaudy "Velvet Elvis", but working on black paper has the benefit of making your pastel colors very vibrant. I like Uart Dark 400 grit and it's very dark but not actually black.
Here's the reference photograph. 
When creating the painting I emphasized the shadows so that the painting would be about 66% shadow and 34% sunshine. It's always better to not have the painting evenly divided between light and shadow. By making more of the image in shadow the sunny areas will become the focal point.

Reference photograph for "Artist and Teacher" Notice I've drawn 1/3 grid onto the photograph

This paper is very dark, so the first layers will be dark, but don't despair. The next layers of color will brighten up the scene. I drew the basic shapes with an orange Conte pastel pencil.

Sketch in the basic shapes. Watch the composition so that the point of interest (artist and teacher) are not in the center.

In step 2 I blocked in the shadow areas and the sunny areas. And in step 3 I blocked in the light sky, the tree shapes and emphasized the sunny area.

Here's an ink sketch of the main subjects - Artists and Teacher. The photograph was taken at a Plein Air workshop with Kim Casebeer who is discussing the scene with one of the participating artists. I liked the way they were sitting in the shadow area with the sunlit area behind them. 

In step 4 I start lightening up the darker areas and using colors closer to the reality of the scene.

The final step shows more layers to get the value correct between light and shadow areas.
 I lightened the background and the large tree to the left of the figures so that area will drop back and add to the feeling of depth and atmosphere. I also lightened the shadow part of the grass so that the darks of the tree shadow would be darker than the grass. You'll see the color swatches along the left side of the pastel. I'm using this image for my Tuesday night pastel class and they will be able to see which pastels I used in the piece.