I bought the vents from www.superskoda.com and got them painted by a local bodyshop.

The obligatory "before" picture

The kit comes with excellent sticky-backed templates

I used a Dremel-type tool and a cutting disc to cut the holes.
 

This is the point of no return. Gulp!

All cut through. Just needed a bit of wiggling to get out!
 

This is me thinking "Oh sh_t, what have I done to my car!"

A bit of work with the sander bit and the edges were nicely cleaned up

All that remained was to glue the vents in place, tape them up, lean a heavy thing against them (bag of fertiliser!!) and wait 6 hours for the glue to fully set
  

I'm well pleased with the results