I did this on my old series 2 with my leg in plaster after an operation so you should have no problems.

Take the old spoiler off ( the screws might be rusted )
Put the 90 spec one on , locate it with the 2 bottom screws
Mark where you will have to drill holes in the higher holes
Take the spoiler off
Drill the holes
Put the spoiler back on
I used self tapping screws and secure it to the body work

This all sounds easy but should the screws be rotten beyond being able to undo them which mine were.
I didn't realise that there is a metal frame in the spoiler and cut V's above the screw locations taking chunks out of the spoiler till I could drill the screws out.

It was a pain in the arse but well worth it