After reading all the horror stories, I was disappointed with how easy mine was when I did it.
These two two threads were very helpful:
http://986forum.com/forums/diy-project-guides/56334-diy-coolant-tank-replacement-under-2-hours.html
http://986forum.com/forums/diy-project-guides/50261-another-coolant-tank-replacement-diy.html
Here's notes from when I did mine:
Lastly, many people have tried to save a little money buying a cheaper coolant tank. Many of those people have posted their regrets.