First I printed the Original in b/w on plain paper (DIN A4 - 80g/m²). Then I painted it with watercolor (aquarelle). After the color was dry, I put a drawing paper over it, disrupted this a little bit and put a layer Scotch Tape over all. At last I scanned it and edited it with Photoshop CS.
For the Sandwich I took the Original and the Layer.
This is the result: