Some tools and guides might require switching your device to DFU (Device Firmware Update) mode or Recovery mode . Instructions usually come with the tools.
Before you start, make sure to back up your iPhone. However, note that backups from newer iOS versions might not be compatible with older versions like iOS 7. You might lose some data or settings.
Get the iOS 7.1.2 IPSW file for iPhone 5s (model A1453, A1457, A1518, A1528, A1530) from a trusted IPSW archive.
Standard DFU mode is not enough; the device must be put into "Pwned DFU" (exploited DFU mode) to accept a custom IPSW.
You need your device's ECID (Exclusive Chip ID) and the specific iOS version you are currently on.