Step 1: Download the Flappy Bird IPA file onto your computer.
Step 2: Download Cydia Impactor from here.
Step 3: Connect your device to your computer using a USB cable.
Step 4: Open Cydia Impactor.
Step 5: Drag and drop the IPA file downloaded in Step 1 onto Cydia Impactor
Step 6: Cydia Impactor will ask for your Apple ID and password, which is verified with Apple only and is used to sign the IPA file. If you’re not comfortable providing it, then create an alternative Apple ID and use that instead.
Step 7: After the app is installed, go to Settings -> General -> Profiles (or “Device Management” / “Profiles & Device Management”). Find the profile with your Apple ID and open it.
Step 8: Tap on the Trust button.
Step 9: Now open the app and enjoy it.