The best is you start with heads of shagnum. For a tray of 60x40 cm you need about 4 litre of them. Drill 2 holes about 3 cm from the bottom. My tray is outside. The holes prevet that the rain covers all the shagnum. The first time it takes about 2 - 3 month till the moss reaches the top of the tray. Then you can cut it with a scissors. The rest which stays in the tray recovers and it starts all over again. Don`t use any soil only the pure shagnum with rainwater.