Press the keyboard backlight hotkey, usually Fn + F7, Fn + F3, or Fn + Space
Look for a key with a keyboard-light icon on the top row
Press the key repeatedly to cycle through backlight levels or turn it on
Check ASUS Armoury Crate or MyASUS settings for keyboard lighting options
Enable keyboard backlight in BIOS if the option is available
Make sure your ASUS model supports a backlit keyboard
Update ASUS keyboard and chipset drivers if the backlight does not work