Before exporting the physical scene, you should

• rotate object in edit mode to align the bound box
• set the origin of geometry to it's bounds center
• apply scale to object data

Update Dec-2020

• Added option to Export only selected objects

Install

• In the Blender program folder, navigate to [[2.8x\scripts\addons]]
• Create a folder in 'addons' and name it something like "io_bullet_blender_exporter" (no spaces, hyphens)
• Copy all the files in this project to the folder
• Start Blender, open Preferences > Add-ons enable "Bullet json format"