You'll need to find a dry, soft piece of sponge with some resistance; the kind used for washing cars is ideal. Use a pen and ruler to draw the exact rectangular shape you would like the rest to have. The width should be approximately 4cm and the length slightly less than the violin back. Use a sharp knife to cut the sponge. Take a strong elastic band and place it over the chin rest. Then take the sponge shoulder rest, hold it in place across the violin back and pull the elastic band across it. Fix the band over the left-hand bout. If you find the shoulder rest moves too much, you can take a second band and do the same, fixing it on the right-hand side.