If you're digging out a hole in the lawn, then skim off the turf first. This rots down into good garden compost if you stack it in a compact heap.

Dig out the hole carefully, making sure that you don't excavate too deeply around the edge where the shelf will rest. Use the marked-off stick to measure how far down to dig.

Once you have excavated the hole, check the base for stones and remove them. If left in the hole, they could damage the liner, because the volume of water exerts a tremendous pressure on the base.

Dampen the sand and evenly spread a 5cm (2in) layer across the base of the hole.