A sunken bowl with a sloping exit does most of the work
Size
Anything from a washing-up bowl upward. Smaller ponds warm and freeze faster but are used by a surprising amount of wildlife within weeks.
The exit
Essential. A sloping side, a pile of stones, or a plank. Without one it is a trap for hedgehogs and small mammals rather than a pond.
What to put in it
Rainwater and one native oxygenating plant. Nothing else. Fish and tap water both prevent it becoming a wildlife pond.
