As others have stated use 1 continuous piece of tape, make sure no tape is seen once installed as dirt will find its way there.
dry fit the spoiler and mark on the trunk with blue tape where it should sit, pull the backing off and go for it with 2 people. Just land it in-between your blue tape marks and your done.
I would also cut the 3M tape to fit exactly. The thin edge should just have enough to cover what touches the car and no more!
09' AW M3, 6MT, Black Novillo Extended, Carbon leather trim, Premium, Moonroof, OEM Carbon spoiler, BMW LED Int. Bulbs, Avant Garde 19x11/19x9.5 M310 wheels, OOLux V3 AEOO, Vorsteiner Type II diffuser, Megan Racing Stainless exhaust, Benvo Stage 1 tune, Dinan High Flow Carbon Fiber Intake, Dinan Front anti-sway bar, H&R Sport springs, 30% tint