Sit the mould on form ply, plastic or a smooth, flat material that won't absorb moisture. Use a scraper or filling blade to fill the mould with cement. Don't fill it right to the top because when the paper towel tube goes into the centre to make the cavity for the soil, it will displace some of the cement. Tap the top with the cardboard tube to remove any air bubbles. Insert the cardboard tube in the centre of the mould, down to the mark. Leave the cement to cure.