Plug the cable onto the conections on the transducer.
Glue the transducer into the tube using hot-melt-glue.
Glue the servo disc onto the tube using hot-melt-glue.
The disk is usually a tight fit on the servo shaft and doesn't need a mounting screw, also if Hextor is to be put in a carry case (it fits into one of the cheap aluminium tool cases on sale in DIY shops) then the transducer will need to be removed for Hextor to fit in.
However if you wish to screw it to the servo then you will need to make two holes in the tube as in photograph 4.
Note the polarity of the connections to the transducer, see photograph 2, +ve to the middle strap at 3 o'clock, -ve to the small tab at 1:30 o'clock.
Mark the connector end (say black -ve) so you know which way round to plug it on the PCB.