Sorry,i forgot to reply in etienne's thread...i can give you a basic idea though!...
First up..it looks like the SS header does not mate up with the OEM exhaust system??,i maybe wrong due to differences in exhaust...there site does say "manifold for OEM Kat",so you may need to check you OEM diameter etc to see if it will slide into the headers coupling...also would be best to see if it lines up with the OEM exhaust!...you could always get the SS exhaust to match!....
also i noticed the SS header only takes O2 from two cylinders!?
to take your OEM header out:
jack the car up and remove the header support from the bell housing and gear box,then seperate the header to exhaust flange,use a box to support the wieght on the exhaust.
Remove any O2 sensors on the header.
Remove the heat shield on the header.
remove the 13 or so nuts holding the header to the engine head.
you may or may not need to remove the SC or SC intake and oil seperator.
on fitment renew any damaged studs and renew the header gasket(as etienne said he had one to go with it)
as i have a right hand drive some things may vary.
Paul