Regular breathing exercises can be a beneficial part of leading a healthy life. Digital games may have the potential to help people practice breathing exercises in an engaging way, however, designing breathing exercise games is not well understood. To contribute to such an understanding, we created Life Tree as the culmination of three prototypal breathing games. Life Tree is a virtual reality (VR) game in which a player controls the growth of a tree by practicing pursed-lip breathing. We selected VR head-mounted display technology because it allows players to focus and limit external distractions, which is beneficial for breathing exercises. 32 participants played Life Tree and analysis of the collected data identified four key themes: 1) Designing Breathing Feedback; 2) Increasing Self-Awareness of Breathing and Body; 3) Facilitating Focused Immersion; and,4) Engagement with Breathing Hardware. We used these themes to articulate a set of breathing exercise game design strategies that future game designers may consider to develop engaging breathing exercise games.