I recommend taking a look at some of the examples that have already been created. The ones of Berlin took my breath away.
The site is very easy to use.
- Find 2 pictures that show a comparison
- best if taken from the exact same spot
- Right click to copy the image address and paste where states URL
- Add a tlabel and credit
- Select preview
- Publish and share
Embedded below is a 1 minute and 19 second YouTube tutorial.