When installing the firing pin in the slide, do the notches on the firing pin face up (toward top of slide) or toward the safety lock?

In this pic he inserts the firing pin with the notches facing the safety lock



But I just watched a video on Youtube and he inserts the firing pin with the notches facing up!

https://www.youtube.com/watch?v=EL4xzwKCByw

Seems like one way is wrong... What is the right way to insert the firing pin.

After looking at it closer, there is only one way it can possibly go together correctly - firing pin notches must face up (towards top of slide).

Correct.  The notch in the safety lock intercepts the notch in the firing pin until the safety lever pushes up on it when you pull the trigger.  When that happens the firing pin can move forward.