The instructions are explicit and often quoted. Human seed is sealed in a vessel and left to putrefy in horse dung for forty days, at which point something begins to move and take shape; it is then fed daily with human blood and kept at the same warmth for forty weeks, after which a small living person is present, fully formed but transparent, and can be raised as a child. The passage appears in De Natura Rerum, printed under the name of Paracelsus after his death.
What it means is contested and has been for a long time. One reading takes it literally as a generation experiment, in a period that accepted spontaneous generation and had no reason to think this impossible in principle. A second reads it as alchemical allegory, in which the vessel, the putrefaction and the forty days are the standard stages of the work described in biological language. A third reads it as a claim about art surpassing nature, made deliberately to provoke.
Two later confusions are worth separating out. The homunculus that Wagner makes in the second part of Goethe's Faust is a literary creation working from this material, not a report of it. And the tiny curled figure drawn inside a sperm cell in Nicolaas Hartsoeker's optical treatise of 1694 belongs to a different debate, about preformation in embryology, and was offered as a hypothesis rather than an observation.