It's pretty easy.
First, open Google Earth(it's a program that you have to download and install, http://earth.google.com)
Second, in Google Earth, right mouse click on the "My Places" link in the Places pane on the left menu. Hover over "Add>" and select "Network Link". Copy/Paste this URL into the link field. Provide an arbitrary name in the Name field. I chose the random name "Sectionals." Click OK to add the link.
It should take a minute to preload the chart data. Expand the newly added "Sectionals" section, which also has a sub-section titled Sectionals underneath it, and select the ones you want to display. Don't choose too many or they'll bog your connection down while they attempt to load.
On my high-speed connection it took probably 30 seconds before the overlay appeared. You might have to be patient, even more so if you're on dialup. Make sure you've panned the map to the location of the sectional so you can see it appear when it's done loading. If you're zoomed out, the sectional image may look fuzzy, but zooming in, while allowing time for the image to stream the image data in and re-scan, you'll notice the resolution improving. At the bottom of the screen there is a "Streaming |||||| xx%" progress meter.
Then click the Camera Angle and lower your 'camera eye' down closer to the surface of the earth (it's a +/- control on the bottom right. Hover to see the tooltips.) You'll magically see the topographical features of your area push thru the sectional chart to make a 3D chart. It is so cool.
One thing that I like to do is set the Elevation Exaggeration to a factor of 2 or 3 in the Options of Google Earth. It makes the topo features much more noticeable.
Here's another screenshot of the Klamath Falls chart overlayed on the Grants Pass, OR area. I pulled the overlay opacity back just a hair to make it a little bit transparent. Click the image for the large version.
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