i used 3.5 x 20mm self tappers form homebase but wilkinson's is cheaper.
This is how i did it (No drill needed):-
- Using a stanleyknife/blade cut the tabs so the splitter is flush with the bumber, soft material so its easy.
- Screw all your self tappers into the splitter first, i started from the middle and worked my way out. This make's it easier to screw straight into the bumper.
- with the car jacked up on axel stands, i got under and held the splitter up i then screwed the centre one in first and then worked my way round. (i had someone making sure it was flush as i screwed).
- Once fitted using the stanley knife/blade cut the ends flush to match the bumper.
- job done.
There is of course other ways to do it, e.g. drilling pilot holes but no need really.