1. Open VRChat
  2. Tap your icon to view your own profile
  3. Tap "Worlds"
  4. Tap a private world
  5. Tap "Instances"
  6. It says "No Instances", with no interface for making any.
  7. Tap the back button
  8. Tap "Worlds"
  9. Tap a public world
  10. Tap "Instances"
  11. There's a "Create Instance" button this time.