Pre-cut pictures, such as butterflies, flowers, spaceships, cars, rockets etc., in the middle of black construction paper. Precut thin black strips.
Let the children paste the black strips randomly over the cut-out butterfly. Using different coloured shapes of tissue paper, they can cover the butterfly shape. All this is to be done on the wrong side.
When completed, turn it over and mount it on a window for the full effect.
Here is an alternative idea from David Orton:
We visited a local church, and the class were impressed by the stained glass windows.
We took some cereal packets, folded one side in half and cut out a window frame (arched or rounded etc.). We then stretched some clingfilm over the window hole to glaze it. The film was fastened in place with sellotape over one piece of the frame. The second half was folded back and stapled into place.
The children were then either given a free design or, according to ability, a drawing could be blu-tacked to the underside of the frame. The children could paint directly onto the clingfilm using acrylic paints. The frames can be painted last using watercolours as this will not stick to the clingfilm.