To boot in recovery mode on a LG G2, you need to do the following steps:

  1. Power off your phone. Don't just restart, but perform a normal power off
  2. Wait for a few seconds
  3. Press <Volume Down> and <Power> buttons simultaneously until the LG logo is shown
  4. Press <Volume Down> and <Power> buttons simultaneously again until you get the firmware reset screen
  5. Press the <Power> button again to start the reset

This is useful when you want to flash a firmware via e.g. TWRP.