Easy job.....
Take the bumper off and turn it upside down. After trimming the two long location spigots (used on Leon) off the splitter, lay it on the bumper where you want it. Now secure it with G-clamps.
Now start in the middle at the front and drill a hole throgh the splitter & bumper, secure splitter to bumper with a nut (nylock) and bolt. Now work your way out doing the same, positioning and tweaking the position of the spiltter as you go.
When fitted you will see that the splitter is about 40mm too long each side in sticks out into the wheel arches. Trim it down with a sharp hacksaw. Refit bumper. Job done.
Bumper removal
guide HERE