Pour the mixture into the metal mould over the caramel, and put the mould (or moulds if you have made various) into a pan of simmering water. Make sure the level of water is lower than the moulds, so that no water spills into the flan mixture. Let the flan cook like this until it is ready. To test, put a sharp knife into the middle of the mixture. If it is clean when you take it out, the flan is cooked.