There are a few things you need to ensure.
1. Mounting holes are in exactly the right position from the edge of the bootlid on both sides.
2. Holes are correct distance from the rear edge of the bootlid.
3. Hole for wiring is drilled in correct spot so that the wiring goes inside the bootlid, not through the double flange into the rain channel.
4. Wiring hole is large enough for wiring to pass through easily and has no rough edges which will wear through the wiring.
5. Holes are properly sealed to prevent rusting or leaks into the boot.
OK, here we go.
Open the boot lid and you will see two small recesses in the inner panel on each side of the inner panel, directly below where the spoiler mounting bolts go. That gives you some idea of the location for the mounting holes.
You should apply masking tape to the outside of the lid, directly over those recesses. Then, if you didn't get any rubber seals with the spoiler, make a template from cardboard to match the mounting leg of one side of the spoiler.
Once you have a perfect fit with the template, including the mounting holes AND the larger hole for the wiring, make a second template, (without the larger hole) and tape them onto the outside of the boot lid, ensuring that the holes are centred over the recesses on the underside and equally spaced from the edge of the bootlid on both sides. Measure this very carefully.
Drill small pilot holes dead centre in each hole in the templates.
Trial fit the spoiler over the holes to ensure that they match the bolts in the spoiler.
Drill out the four mounting holes to the diameter required and dress off each hole to remove any burrs.
Drill the wiring hole at a slight angle, with the drill inclined inwards to ensure that your hole exits inside the inner skin. Open this hole out with a larger bit and carefully dress off all edges.
If you have rubber seals, it's just a matter of bolting the spoiler down, first of all running the wiring into its hole and feeding it through as you bring the spoiler down onto the bootlid. Make sure the wiring is pulled through completely and not caught between the mounting and the boot.
The wiring connects to a plug already clipped inside the boot. It sits up just inside the inner frame, slightly to the right of centre and near the wiring to the boot release.
Hope this explains it for you. The trick is to be careful and make absolutely sure the holes are evenly spaced from each side and from the rear edge of the lid, and match the spacing on the spoiler. Get that right and it's plain sailing.