Note: This is basically a placeholder for a future breakdown. It still has some resources for you to dive into in order to construct your own but it's not a step by step write up.
The basic premise is using a conductive rubber cord that shifts in resistance when it is stretched and retracted. We cut a small piece, attach both ends to a belt and an arduino and we can then monitor the shifts in resistance to feed breathing data to a future Unity program.
Breath Sensor – Non0Mad
Basic Belt Respiration Sensor by erin_gee – this is the step by step that will help you build your own.