yup, that's pretty much it. I think on the underside of the spoiler it's not completely flat, i.e. when you rest it up against the tail gate, so you should put the silicone on all the bits that actually make contact.
The araldite will set hard after 20mins, so you'll have sometime for adjustment to get the spoiler sitting perfectly. Use the gaffer tape to hold it in place. After a few hours check its stuck firmly, within a day mine could take some pretty hard knocks, i.e. I could lift the back end up slightly by pushing up on the spoiler, but I wouln't push it too much, might end up ripping the paint off.
