Once the app appears on your home screen, you won't be able to open it immediately. Go to Settings > General > Device Management (or Profiles). Find the developer profile and tap Trust .

Ensure your device is connected to Wi-Fi and has at least 50% battery.

Socket (installs the modern Zebra package manager) Step-by-Step: How to Use Jailbreaks.app Legacy

EverPwnage (a modern alternative to Phoenix) iOS 8.4.1: Daibutsu and Home Depot

The Ultimate Guide to Jailbreaks.app Legacy.html: Reviving Your Older iOS Devices