To get started, fire up Google Earth, zoom in a bit and then press Ctrl+Alt+A (Cmd+Opt+A for Mac users). You'll be presented with a screen like this:
Here you can select one of two planes (F16 fighter jet or SR22 prop plane - it doesn't seem to make much of difference which you choose) and select the airport and runway you wish to take off from. Make your choice and click "Start Flight". Soon you'll be sitting on your selected runway, ready to take off.
Now go flying. In order to take off, you need to build up some thrust - press the Page Up key for a few seconds (the thrust meter is the left bar of the lower-left meters). Once you've accelerated to a sufficient velocity use the arrow keys to take-off. The keys are in reverse, as one would expect with any flight simulator, so use the down arrow to take-off. Just be careful, it's very sensitive. Once you're airborne, you can start using your mouse to control your plane.
Anyway, soon enough you'll be off the ground and flying over the country you selected to take off from. I took off from Heathrow, flew for a bit and then doubled back on myself just to see how accurate and clear the view would be...
... and I was impressed. If you crash, don't worry, you can select to continue with your flight and Google Flight Simulator will then start again, but with you in the air near where you crashed.
If you decide to quit, Google Flight Simulator doesn't return you back to the normal Google Earth view, instead you remain in the "Simulator" view (it's normal view, but with a lower level of elevation) and you can then use your mouse to move around and view things as if you were flying, but without the need for skill. I thought I'd go check out Windsor Castle:
I'm pretty impressed with this hidden feature. I wonder how long before they advertise it as official functionality. Who needs MS Flight Simulator when you've got Google Earth?