Whether you download a historical map from an online digitized collection such as the Warrantee Township Maps available online from the Pennsylvania State Archives, purchase a historical map on CD, or scan one yourself at home, it is really easy to overlay the map on Google Earth and see your ancestor's property as it exists today.
To begin, all you need is a digital map image of some type - saved as an image file. Permissible file types include the most common graphic file types such as JPG, TIFF, GIF, BMP and PNG, as well as PGM, PPM, DDS and TGA. This can be something such as the Pittsburgh Warrantee Township Map that I used here, a land plat you have drawn yourself, or a historical map scanned from an atlas.
If you don't already use Google Earth (not Google Maps as the functionality is different), then you will need to begin by downloading and installing the free Google Earth software, available for both Windows and Mac. Then just open a new file and zoom in to the city or other location where you want to place your map overlay.
Next, click the "Add Image Overlay" icon and browse to the file on your computer that you want to add. When the image displays, leave the editing window open (you can minimize it to get it out of the way) and then use the green crosshairs in the center, and on the edges and corners of the picture to move it around, and resize and stretch the map until it matches with at least one geographic feature that would still look very much now like it did then. In my very basic example, I used the Allegheny and Monongahela rivers in Pittsburgh to fit my map, but even then the river bends didn't match exactly - this could be because the river courses have changed a bit over the past century, or because the Warrantee Township maps weren't drawn exactly to scale.
For best results when positioning your image overlay, use the Center crosshairs to first position the image, followed by the triangle marker on the left side to rotate the image. Then use the corner crosshairs and side anchors to stretch or skew your map for the best fit.
The Edit Image Overlay window also allows you to adjust the opacity of the historical map, dimming the overlay until you can also see the present-day terrain underneath. Once you close this window, you can get back to the opacity setting by selecting your overlay map in the left-hand viewer. You can also right-click (Control click on a Mac) the overlay in the Places pane and select "Get Info" or "Properties" to bring back the Edit Image Overlay window and the image crosshairs.
Google Earth's tutorial on Overlay Features offers additional tips and tricks, including suggestions for positioning your overlay, forcing your overlay to integrate with the terrain or shape of the land beneath it, and even email your finished overlay to family and friends.
If you think this sounds like too much effort, then you may be in luck and find someone else has already done the hard part for you. Several historical map overlays, including the Rumsey Historical Maps layer, are available as a layer directly in Google Earth. Alternatively, there are websites where you can find free downloads of historical map overlays sized, georeferenced, and ready for you to import directly in Google Earth.