The sachet (1) is made from two identical supple sheets which are joined round their periphery. It is held in a closed chamber. This has a lower part (2), that holds the sachet, to which an upper piece (8) is clipped. The method includes introducing a mixture of air and water, under pressure, into the chamber, through a hole (10) in the upper part. Spikes (11) on the base of the upper part pierce the upper surface of the sachet. Its lower surface is pressed on to points (19) on the lower part. These have holes (20) between them through which the liquid flows.