These are built by sandwiching a piece of porous, spacing fabric (e.g. tulle) between two sheets of piezoresistive fabrics (from Eeonyx). For the square pad depicted below rectangular sheets are placed at right angles and wrap around the frame. On the inside they are stapled to a conductive strip on each edge. The four edges are wired to the analog inputs of a suitable microcontroller, e.g. uOSC, Teensy or Arduino.
For sensitive touch use the Eeonyx resistive fabric made from spandex.
For heavier touch consider a nonwoven conductive fabric such
These fabrics are available now from Adafruit, SparkFun and others now.
Below is also a video of Diane Douglas demonstrating two of these pads she built according to the model shown at Maker Faire.