The first thing I did was draw out my underwater scene in oil pastel. I made sure the lines were really thick and chunky so that they'd have enough body to push away the water once I painted on top of the drawing.
Then you can spread the paint thinly over the whole image. Don't be worried about going over the top of your original lines as the oil in the pastel will push the water in the paint away. I decided not to blend in my brush strokes too much, as I thought they gave a bit of a wave texture.
Once the blue layer was dry, I got some undiluted acrylic paint and painted in the bodies of the sea creatures. This made them really pop out from their surroundings.
I then decided to finish of the piece by drawing and cutting out some frame-style shapes on another piece of paper. I coloured and decorated these shapes with oil pastel.
These pieces could then be glued on around the edge of the image.
Oil pastel lessons are always a winner with the students, so I'm sure they'll have a lot of fun creating their underwater scenes!