If the spoiler is a Honda spoiler, they provide very usefull templates... Its a matter of finding where the spoiler will sit on the trunk, which can be done easily by measuring the span of the bolts on the spoiler... then place the templates on the trunk and measure the distance between the respectable "holes" on the templates... and measure from the front of the trunk lid, to the front of the template on each side, so both ends of the spoiler sit in the same place. Lightly center-punch the holes through the template... if you hit the punch too hard, you'll dent the steel around the hole and the spoiler gasket wont seat properly... also, when drilling the holes... start with a small bit, and gradually enlarge the hole by stepping up the bit size each time, being sure to drill slowly to not damage the surrounding steel... if you drill too quickly the bit will catch when it gets almost all the way through and "twist" the surrounding steel... so just take your merry time and be careful.

You know when you get a real Honda spoiler, cause the directions sometimes are in Japaneese.