This was one of the first mods i did on my s3 and in my opinion is a great addition to the front bumper, nice and subtle.
When i bought mine i found it was cheapest to buy from the seat dealer. Think it was about £40 ish but may well have gone up now.
I simply just jacked up the front end of the car keeping it level with axle stands (you want plenty of room under there for drill)
I mocked up the splitter first taping it to the bumper so i could get it dead center, using self tapper screws i worked my way from the center out, keeping it flush to the bumper and level all the way round. I would personally not skimp on the screws as the splitter needs to be secure at high speeds. When you get to either end of the splitter you will need to do some trimming. I used a heat gun and a decent cutting blade to smooth off and trim the splitter so it fits flush to the underneath of the bumper. That's it really. Just take your time and make sure your happy with the position before drilling pilot holes and fixing etc.