Basically, you draw a picture and then lightly color the whole picture with watercolor pencils. You don't have to color exactly. When my son and I practiced this, it was easier to 'paint' if you colored basically the entire area you wanted to paint. Then grab a paint brush and water and 'paint.' You don't have to color perfectly, because once you add water, you will be blending the pencil strokes with the water and paint brush.
I used a dark blue and light blue and a dark green and light green to achieve sky and grass variations.
So, grab some watercolor pencils and start coloring.
Here are some other pictures I did. If you can't tell, I can't draw worth a hoot.
When I tried heavily coloring around the edges, it was more difficult to spread the paint to the white areas. The other ones turned out much better when we lightly colored the entire area.
Here they are painted.