I took the bumper off to fit mine (much easier access but a bit excessive), would be easier to jack up the front and put it on axle stands.
The megane splitter is just about the right width but too long on the ends, so no bending required really (a little at the corners but i didn't need to modify it) just need to chop off about 3" from each end (I used a junior hacksaw after i'd fitted it).
To actually fit it i got lots of self-tapping screws from B&Q (can't remember the size but only need to be 1cm long at most), about 30 of them for £2-3.
Line up the splitter and attach loosely with masking tape. Using a 1-2mm drill bit, drill a pilot hole throught the splitter and bumper (i started in the middle of the splitter) and screw in a self tapper.
Continue every 6" or so, can add more screws anywhere it's not fitting quite right.
Saw off ends, et voila!
EDIT: Was a while ago that i did this, so ask if I've missed something out!