I started with a pen drawing of jellyfish, then painted their inards with a Liquitex Interference paint. I had some fabric [transfer print which I did at college a few years back], which I scanned into the computer and then printed onto a sheet of translucent vellum. I then drew on similar jellyfish to those on the sketchbook page.
One is then placed over the other to make it look like a sea full of jellyfish.