On the other side of the paper on which I drew the Iris, I completely blackened with a 6B pencil. Maybe a graphite stick would do a better job at covering the area quickly. Then, with the right side up on the watercolor paper, I went over the complete drawing. It came out quite light, I could barely see it and so I went over the lines on the watercolor paper again. If you use too thin a paper for your drawing, you wont be able to see the lines properly when you blacken the other side. Here, you can see the other side of the iris drawing paper at the top of the photo.
Watercolor Journal 
