After laying in a rough background I begin cutting out the foreground. In a layer under the drawing using the same brush and a Hard Eraser I cut out the silhouette of the binyup and branches, right up to where they meet the water. I drop the sketch onto this layer, clean up extraneous lines and lock the Alpha (Layer>Lock Alpha). This can be used as a mask. Now I’m free to use larger brushes to scribble around but not spill over the edges. I’ll turn this on and off as I work.