How do I install this Ribble endurance SLR headset?
In my experience I believe you should start with items in https://i.stack.imgur.com/so4gwm.jpg initially. Starting with the thinnest one first, in the direction facing down as you have them in your hand.
Lastly using this on the top: https://i.stack.imgur.com/cT9ppm.jpg
I would recommend using the Parktool Youtube page for more detail on Headset installation:
https://www.youtube.com/channel/UCzaZ1sPWEuZN-I8_XT6AH8g