I'm not sure this is the correct way to do it, but based on results, it seems to work.
Firstly, these particular lights weren't as bad as the other ones I had (have 2 cars right now).
Here's what the light started life as.
(https://i.imgur.com/9lKNf3v.jpg)
IT's not the worst, but it got me an advisory on my recent MOT.
QuoteRepair as soon as possible (minor defects):
Front Headlamp lens slightly defective Both (4.1.1 (b) (i))
Things required:
wet and dry paper.
600 grit
1500 grit
2000 grit
2500 grit
3000 grit
meguiars ultimate compound
DAS6 polisher or elbow grease!
Polishing cloth
K2 Lamp Protect
About half hour per light.
The method is pretty straight forward. Wet all the wet and dry and start with the 600 grit paper.
Make sure it's really, really wet. Lots of water.
It'll scare you!
(https://i.imgur.com/1bMrq5a.jpg)
Once you've got rid of all the yellow and any discolouring, move on to 1500 grit.
Make sure you take your time and plenty of sanding and water with this, then the 2000, 2500 and finally the 3000.
With the 3000, to ensure I didn't get any swirls, I did this with almost running water and did it for quite a while.
When you've got this far, you'll think "but it's still cloudy"
Next up, get your "cutting compound" (as I said, I used Meguairs - G3 didn't work for me) and polish.
I have a DAS6 (Dodojuice in case you wondered). I used a pad to put the compound on - as you would with paintwork. Then polished with the DAS6 before finally giving a polish with a microfibre cloth.
Put the K2 on as per instructions. Put it on with the provided cloth then leave to dry. 24 hours later, do it again. Finally, leave to cure for a week (i.e. don't clean your lights with anything else).
Here's the difference
(https://i.imgur.com/Ymk261r.jpg)
And finally, when both are done.
(https://i.imgur.com/FkV6hrV.jpg)
And in case you wondered if you can take them out and do them, they also come out very nicely.
(https://i.imgur.com/af4lLfV.jpeg)