Assembly is pretty straightforward. Solder the plus and the minus cables of the audio cable to the piezo element. Superglue the piezo to the glass. If your using a small glass pane, duct-tape or glue the glass to the picture frame.
Be careful not to get any superglue on your skin (never, like I once did, try to open it with your mouth), if you do, get it off directly.
Stick the minijack in the microphone input of your computer and put your speakers on: now you can listen to the rain.
The file attached to this step is a wave file of a minute recording of rain (quite a heavy storm).