After his open - air observations he returns to the studio to add a different sort of color, color made from pieces of paper in generic shapes. (painters-table.com)
The artist drew on sheets of paper in his lap, completing several compositions in one sitting. (brooklynrail.org)
If you can't find someone to learn from in person, don't be afraid to email the corresponding author of a paper with the technique you want to replicate. (ascb.org)