Co-design and youth participatory action research are promising methodologies for increasing youth engagement in well-being interventions. The current study included 10 recent high school graduates employed as youth advisors to co-design a youth-friendly positive psychology intervention targeting the post-school transition. The youth advisors received foundational training in positive psychology, psychological needs theory and emerging adulthood. They held regular meetings to develop a Positive Transitions programme (PTP) text messaging intervention, informed by discussion of key themes, issues and relevant practical tips. This paper aims to examine the co-design process from the perspective of the youth advisors. Each youth advisor completed a survey, and six of the advisors also participated in a focus group exploring their co-design experiences. Youth advisors noted a range of personal benefits from the co-design experience including learning, enjoyment, teamwork, achievement, and pride. Areas for improvement included clarity of the project scope and the provision of support and feedback. Insights from this process are discussed, with the aim of guiding future work seeking to design youth-friendly well-being interventions.