We explore 360 paper prototyping to rapidly create AR/VR prototypes from paper and bring them to life on AR/VR devices. Our approach is based on a set of emerging paper prototyping templates specifically for AR/VR. These templates resemble the key components of many AR/VR interfaces, including 2D representations of immersive environments, AR marker overlays and face masks, VR controller models and menus, and 2D screens and HUDs. To make prototyping with these templates effective, we developed 360proto, a suite of three novel physical--digital prototyping tools: (1) the 360proto Camera for capturing paper mockups of all components simply by taking a photo with a smartphone and seeing 360-degree panoramic previews on the phone or stereoscopic previews in Google Cardboard; (2) the 360proto Studio for organizing and editing captures, for composing AR/VR interfaces by layering the captures, and for making them interactive with Wizard of Oz via live video streaming; (3) the 360proto App for running and testing the interactive prototypes on AR/VR capable mobile devices and headsets. Through five student design jams with a total of 86 participants and our own design space explorations, we demonstrate that our approach with 360proto is useful to create relatively complex AR/VR applications.