I got his sensor at Maplin two years ago in the UK (there are no Maplin shop in the Netherlands) but did not look at it for a while, until I had some time to have a look at it this Easter.
Mount the sensor somewhere under a laminate floor or a doorstep (I gave it a small 3D printed pillbox to somewhat protect it). But as soon as you do this then the signal becomes really small, furthermore there is a cable generating noise between the piezo electric sensor and the rest of the project, in other words, you need to condition the signal. Eventually I empirically designed this (all comments are welcomed):
Finally you put everything in a 3D printed case, this was my 2015 Easter vacation project. On the router at the PC you get to see when someone passes the sensor.
And here are two screen shots for the case and the pillbox to protect the pressure sensor. All made in google sketchup which the best free CAD software there is in my opinion.