make sure you clean the area well, don't use the adhesive supplied as its crap. Once the spoile is painted mark up where the spoiler needs to fit around the brake light, once happy with the position put the adhesive on the spoiler following the instructions then push into position, try an get it in the right place fits time. Once on use some low tack masking tape to hold it in position, i put some bits around the top of the hatch to pull it closer to the car. There is quite a bit of flex in the spolier to take up. I also put some kitchen roll between the spoiler and the tape so that the glue in the tape did not react with the paint on the spoiler.
You then need to leave the car for at least 24hours before driving and don't wash it for a few days with the current weather it should stick quickly. It is quite easy to do.