When I did my spoiler, I covered the relevant area of my trunk with green painter's tape. This allowed me to line up the spoiler and draw all over the tape (and then measure my lines) until I was satisfied with the fitment. I definitely found it easiest to line the spoiler up to the brake light. Then I traced some outline points and measured them carefully. This was the oem-(ish) trunk lid spoiler.
Then when I was going inside to get my drill bits to drill the hatch, I dropped the spoiler on the concrete and scarred up the edge pretty bad
I'm happy it was just a knock-off, so I'm only out a little less than $200
