It's fairly easy.
There are several website that have done some of the important raids already and provide a link that opens up a file in Google Earth.
In Google Earth pull into the appropriate area/town your interested in, click "Add", then "image overlay".
You then get a new window opening. Choose "browse" to locate your image, name it or it can get lost.
Then have hours of fun rotating, resizing your image using the green markers until it fits the Google map. There is a "transparency" slider bar that is very useful.
I have entered all my Dad's missions over the Italy and the Balkans during WW2. I have his original target photos.
Dad liked Milan so much they went back twice in one night!
I have done some for a few other people as well.
Lancaster raid on Bergen.
They look a little static on here but on Google Earth, using the transparency bar it is quite revealing to show the town underneath as the image is made transparent.