1. Take some infants and rip them from their mothers.
2. Bring them up so that they want nothing, but never speak a word to them. Not a single recognizable utterance. Be careful of onomatopoeia.
3. See what language they naturally begin to speak.
Entirely unethical—actually, barbaric. And the modern conception is that these children would not speak a damn word—there is no innate language, only an innate language ability (and perhaps, following Chomsky, an innate sense of grammar). They may in fact concoct a small pidgin but would be psycholinguistically stunted, and the later the experiment ended, the more severe and permanent the disability.
Legendarily, Pharaoh Psammetichus ordered the experiment done and in due time their subjects uttered the word bekos, taken to be Phrygian (XPG) for bread, which shocked the ethnocentric Egyptians. Another story has James II of England getting Hebrew-speaking children.