Once you've marked out an outline, it's time to start digging, taking care not to loosen more soil than necessary. Dig out any shelves from compacted soil - don't attempt to build them up later from loosened soil because it won't work.

Having double-checked that the excavation is large enough to accommodate the thickness of clay, begin lining the base with clay. Keep it wet and workable, treading the air out of it as you go. Some people make wooden implements or use the edges of planks to knead the clay with.

Once you're satisfied that you have a sealed base, move up to the sides, never letting the material dry out. This means you'll be sloshing around in a footbath, bucketing the water from around your feet to keep the sides wet. Keep going until you reach the lip of the pond, which must be attended to just as diligently. Bring turves, topsoil or slabs to the edge and fill the pond to the top.

You'll now have an extremely murky pond containing suspended particles of clay. This will take many weeks to settle, and won't ever settle if the clay sides become exposed, because rain will wash more particles into the water.