I did mine just a little while ago. I was painting them on so I had to have it right the 1st time. This is what I learned. 1: Your headlights are probably not the same height or angle as mounted. On my 7, the passenger side inner corner sticks out much further than the drivers side. I made metal templates of the proposed eyelid and discovered that the angle difference affected how the eyelids appeared from about 10 feet away (passenger side appeared lifted). I also found that the templates were not a dependable method as there placement was subject to movement upon multiple removal/applications. I suggest you get some blue painters tape (narrow), and some sharpies (3 different colors. they will wash off the lexan so no worries). Also a ruler with an end catch. metric preferred.
Measuring accurately and replicating on the opposite side can be a challenge depending on how curvy the headlamp is. I found that putting down some tick marks on the bottom and sides was helpful (use the ruler end catch to 'hook' the headlamp edge). Once you have some measured ticks mark down, you can then play with the painters tape to find the amount of eyebrow you want, and be reasonably assured that you have both sides equal (or at least split the difference between appearance and accuracy). It took me about 3 hours to figure this all out which really is not to bad I guess (I spent an hour dorking with the templates). Notes: 1:Do not trust paper implicitly. It can stretch. So if you put a tick mark on the tape say, remove and reapply later it may be off a tad. Just expect this. 2: If you spray, insure you have LOTS of drop cloth coverage for where you do not want the paint to go. norcalfocus01 has a thread (sticky?) discussing this. 3: Depending on color, insure you have an acceptable color match (one benefit of metal templates was I could paint them and check this 1st).
That is the highly accurate >anal< method.
Edit>Add> It is also best practice that, once you have your paint lines drawn, remove the headlamps for painting. This negates the need for all the drop clothes and also insures you get a nice clean paint line near the seals.