Phone in your pocket or hand, walk for 30 seconds. Accelerometer peaks become steps; the timing, symmetry, and bounce pattern of those steps becomes a fingerprint that's surprisingly stable across walks — and surprisingly different between people.
Hold the phone consistently. Same orientation each time (e.g., always portrait in the front pocket of the same trouser leg). The fingerprint is the combination of you and how you carry the phone, so changing pockets changes the result.