It's really simple just takes a little while sanding I did it on my lude a few days ago and it worked really well
1) Remove your headlights
2)Wet sand it down evenly using high grit depending on how dirty your lights are. I started out with 800 grit wet sanding it evenly (This would take about 10mins each highlights on each grit level) , then moved up to 1000 grit, then 1,200. Using low grit makes it look really bad at first you might even see scratch marks but this will be eventually sanded down also by the higher grits.
*Note using higher grit sand paper will make it look better but make sure you start from the lower grit because starting out with higher grit wont get you anywere*
3) Once your done using your finiest grit paper wash off the dirt/sand and use a plastic polisher which can be purchased from any hardware store. I got mines from G.I Joes.
4) Shove back your headlights and high beam the next person you see to show off your new lights