You get a nice bit of bubble gum, say hubba bubba, then spread it along the splitter, then push it up agianst the bumper, that'll hold it
Seriously, you need to raise the front end up a bit, get a mate to hold it against the car, get under the front bumper, mark out where you want to screw drill a small pilot hole to start off then go a touch bigger before screwing into it.
You will probably need about a dozen (or more) self tapping screws.
It will be a lot easier if there are two of you on it, rather than just you.
If it has to be cut, work out how much you need off while your mates holding it up to the car, cut from the middle, you'll need to clean up where you cut as when you put them together it'll look ugly otherwise.