Go to Settings - General

Tap on Bluetooth, if its Off, tap on it to turn it On.

Now put your bluetooth keyboard in pairing mode simply by turning on the keyboard

Once you do this, your keyboard will show up under Devices and say Not Paired.

Tap on it, You will be prompted with a message to type a series of numbers on the bluetooth keyboard and then press Enter on the keyboard.

The keyboard should up under Devices and say Connected.