I used the writeup by Mark955 on AW, but thought I'd add some pics for the more visually-oriented =)
Thermostat is part # 06B-121-111-G. my original was a -D, and I believe G is being replaced by H (as per my invoice).
the operating area:
Step 1: Drain coolant (sorry no pics)
Remove bottom belly pan. Remove coolant reservior tank cap. Remove plastic clip holding coolant sensor in place at lower radiator hose/pipe. With bucket/catch pan underneath, remove sensor and let coolant drain.
Step 2: Remove upper radiator hose
remove the small hose connecting the coolant reservior to the upper radiator hose. Then remove the larger hose from the hard pipe running along the intake and from the radiator. You'll need to pop up (but not off) the metal clip holding the hose to the radiator, then firmly pull back:
Step 3: Remove intake hose (throttle body-to-intercooler)
remove driver's side headlight (not necessary but makes it a lot easier). Loosen clamp at each end and remove hose.
Step 4: Remove throttle body
remove electrical connector and 4 hex bolts (5mm):
Step 5: Loosen serpentine belt and slip off of alternator
use a box-end or adjustable wrench on tensioner, pull to the right. Carefully slip belt off alternator (towards front of car). Allow to hang loose below
Step 6: Remove alternator bolts
remove 4 bolts holding alternator to bracket:
Step 7: Remove electrical connector and grounding cable from alternator
after removing the 4 bolts, lay the alternator face down on the top of the fan and remove the electrical connector, and the grounding strap. If you haven't disconnected the battery, be sure to cover the grounding strap in tape so it cannot touch other metal surfaces
alternator removed; grounding strap shown:
Step 8: Remove vertical support piece from intake manifold to block
remove the two bolts holding the support piece (6mm hex):
Step 9: Remove bolt from metal hose on t-stat; remove bolt from top of oil cooler
this will allow you to move the hard pipe away from t-stat. On my car, even after removing the bolt I was unable to remove the hard pipe at this time - I had to remove the thermostat from the block, then wiggle it free from the pipe afterwards
bolt holding the hard pipe on the right, two bolts holding the t-stat to the block on the left:
bolt holding the metal pipe to the top of the oil cooler (as seen from top of intake):
Step 10: Remove upper and lower hoses fom t-stat. Remove electrical connector on back side
Step 11: Remove 2 bolts holding t-stat to block, and remove t-stat
Step 12: Install new thermostat
install new thermostat, making sure to remove the old O-ring. The new t-stat will include a new one. I wet my finger with coolant and ran it along the O-ring to pre-lube it before installation. Otherwise, install is reverse of removal:
All done - looks just like when we started!
Disclaimer: this information is for informational purposes only. I am not responsible if you somehow manage to blow up your car in the process.