Direct 5V supply: just add a diode...

Arduino documentation states that you should not bypass its regulator to supply power - i.e.: don't supply 5V on the 5V pin.

I just read that you can bypass the onboard regulator - just add a diode from 5V (anode) to Vin (cathode) to avoid reverse driving the regulator; for example an 1N4001.

Current through a diode can only flow from the anode to the cathode. Usually diodes have a black or gray line near the cathode.

LEDs: "light emitting diodes":