Which (Windows/Mac) are you using to manage the device? Is your iOS 7.1.2 device already jailbroken ?
With AppSync installed, use one of the following desktop tools to transfer the patched IPAs from your computer to your iOS device: ipa files for ios 712 patched
If an app immediately crashes, it usually means it hasn't been properly decrypted. Ensure you have AppSync Unified installed correctly. Which (Windows/Mac) are you using to manage the device