The onboarding phase of Free-to-Play mobile games, covering the first few minutes of play, typically sees a substantial retention rate amongst players. It is therefore crucial to the success of these games that the onboarding phase promotes engagement to the widest degree possible. In this paper a set of heuristics for the design of onboarding phases in mobile games is presented. The heuristics are identified by a lab-based mixed-methods experiment, utilizing lightweight psycho-physiological measures together with self-reported player responses, across three titles that cross the genres of puzzle games, base builders and arcade games, and utilize different onboarding phase design approaches. Results showcase how heuristics can be used to design engaging onboarding phases in mobile games.