remove the plastic lens first -- it is pushed into the rubber and uses the pressure to stay in place.
I...and most other guys I have seen ... are moving them to the plastic just in front of the gas cap.
USE THE BACK OF THE LENS AS A GUIDE --- IT IS NOT A CIRCLE.
remove the plastic off of the tank
take your time --- start with drilling a smaller hole -- I did that -- then used a file.
When you get it close -- check it. Don't go TOO big -- a little at a time.
Good luck