Well.. Everyone has their own idea on how to proceed with this but I personally laid out my template drilled acouple of holes in the corners and used a hand fine toothed keyhole saw. I would have used a jigsaw with a fine toothed blade but I broke it earlier
Ive done 3 sets of headlight in the last 2 weeks. The biggest issue i found was making sure you dont get a gap on the top. Depending on the type of lights you are using will depend on the difficulty. Th eones I used had a blinker in them as well and were pretty difficult. I used a dremel tool anfd a file to get the fine details. I will tell you that the blinker side is where you want to make a mistake if you are gong to, it covers the gaps well.