The guide is pretty thorough but just to clarify:
Power down and remove the power lead before starting, NEVER make any internal hardware changes with the power cord connected.
The PCI-e slot may have a different locking tab, some are located at the top ( as in the guide ) some are at the end, those at the end should be pushed down, towards the motherboard to release the GPU. In both cases the tab must be HELD down to release the card.
Be sure the system power supply can handle the upgrade, does it have the correct PCI-E power cable/s? Does it have enough output? If it lacks the required PCI-E power leads it should be changed for one that does, you CAN use molex/PCI-E adaptors but it's a very bad move, especially if the unit is borderline on output.
Only download the drivers directly from the manufacturer website, nowhere else.
Because you're staying with the Green Team you don't really need to uninstall the current drivers, just use the 'clean install' option when you run the driver package, it should be in the options window.