I would make your self some templates of the mounting bolts first, then do some maths, to ensure it is dead centred etc, mark out the boot lid as will be hidden when once on anyway.
No hurry so take your time, check for aerial mast clearance, wiper arm clearance(if fitted).
When you start drilling use a centre punch, or nail to mark the centre, then progressively enlargen holes as needed.
Once all the holes are done, then file/remove all rough edges around holes, then apply anti rust compound, a few coats allowing to dry between each, then pain any old paint over this, spray paint into the lid and use small modelling brush.
When all dried put some sealant, such as Loctite 5900 or 5910 to both sides of hole edges, this will make doubly sure of no rust, also apply sealant to the legs/supports/sections of the spoiler where it attaches to the car, bolt down, allow 24 hrs to fully dry sealant, then simply pull the sealant it will come off neat as anything.
That's how I do it, to ensure a good fit with no rust in a few years, remember preperation is 90% of the job for a good job.
Post some piccies up once done!!
tabetha