1. If you haven't already watched a cylinder replacement video, search for "Office Chair Cylinder Base Removal" on YouTube (http://www.youtube.com/watch?v=uiDby4FouwQ) to get a general sense of what you're supposed to do. Customer Service recommended this particular vid.
2. Of course, it looks easy in the vid. But the bottom of the cylinder can be hopelessly stuck in the chair base. Customer Service recommended pulling the "pin" straight out from the bottom of the stuck cylinder. In reality, it had to be pried and pulled (and destroyed in the process) with a screwdriver and pliers.
3. Once the pin is off, the gas cylinder comes apart fairly easily and the chair seat with the top part of the cylinder separates from the chair base with the stuck bottom part of the cylinder.
4. The top part of the cylinder can be removed from the seat with a pipe wrench. Tapping it to cause vibrations and so loosen the cylinder top and work in the WD40 before trying to remove it with the wrench helped.
5. The stuck bottom of the old gas lift can be removed by pounding with a mallet on its bottom (pound toward where the seat used to be). It is much easier to remove than when the cylinder is intact.
P.S. If you bought your chair before August 2009 and have not replaced the two bolts securing the seatback with the stronger ones provided under this recall http://www.cpsc.gov/en/Recalls/2010/Raynor-Marketing-Recalls-Quantum-Office-Chairs-Due-to-Fall-Hazard-Sold-Exclusively-at-Office-Depot-Stores/ I strongly recommend doing so. My son's chair broke when he leaned back on it and there are numerous reports of falls and near-injuries on the chair's Office Depot review pages.