In a metal flan mould (you can buy them in most Ferreterías or in the kitchen utensils department of a department store or large supermarket), burn the desert spoon of sugar with one dst spoon of water. Stir over a strong heat until the sugar and water becomes a brown coloured caramel. If you want to skip this step, you can find ready-made caramel in most Spanish supermarkets.
Beat the egg with the 2 desert spoons of sugar. Beat well, add the glass of milk, and beat some more.
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.