Place bottom of container on sponge and draw round it. Cut out the shape.
Wet the sponge, ring it out and place it in the container.
Place the container on the greaseproof or oven paper and draw round it again. Cut out the shape.
Place the paper on top of the damp sponge in the container.
Mix up your colours on your normal palette (I use an old sheet of glass) and scoop them into the stay-wet container with a palette knife.
Close up the container when you're not using the paint and it will stay useable for weeks!