This thread's a bit old, but I found it to be very good--so here are some of my tips and another photo or two.
1. I did not remove the valence, but did get the front of the car up on jack stands. I was able to get under the car alright without jacking it up. I wasn't able to drill the holes (not enough clearance for the drill and bit) without it on stands.
2. Definitely remove the protective cage and bar. There's no way you could possibly get this job done (with the valence on the car) without taking these parts off. I used this opportunity to remove some surface rust and paint black satin.
3. Small vise grips are your best friends in this job. I tried the bulldog clip technique as others suggested and found them somewhat useful, but the vise grip could really pinch the rubber and the valence together like nothing else. Its also very easy to tighten and loosen them. Here's a pic so you know what I mean--I used both types in this picture, but the one on the right was best to use to hold things in place to drill. I moved them as I went from hole to hole.
4. This thread recommended using a 5/16" drill bit in step 5. I went a different route--5/32" bit for a pilot hole and then a 1/4" bit for the final hole. This is very close to the size of the bolts.
5. I put blue painters tape on the sides of the valence before making the marks and drilling the holes. This prevented the paint from getting scratched up while I triple-checked the hole alignment and made me less concerned about slipping while drilling.
6. Check your valence for dents or a bent interior lip before starting. I had a dent I should have seen on the driver's side. I figured it out, but if you have a bad one, you might find the rubber won't align properly or the bolts won't be long enough.
7. Wayne's book was the only source showing the proper hardware order.
8. It doesn't look like the removed A/c protective parts will go back--they will. Here's a shot with everything back together.
9. Lastly, I'd gone ~10 months with only the rear Carrera tail installed. For those doubting it, you can feel a huge difference in the car's handling after installing the front spoiler--and that's just on the road.
David