ILS works with radio waves, just like voice communications, or other navigation aids like VOR. This means a specific approach will use a specific frequency for its navigation signal.
When the pilot is told what approach to expect, they will consult the chart for that procedure for the information needed to fly it. The top left corner of an ILS approach chart from the FAA will show the frequency for this approach:
The pilot will tune the navigation radio to this frequency (or the FMC will do it automatically). Where they do this will depend on the aircraft. On the 737, the radios are tuned on the center console, just behind the throttle quadrant. The navigation radio tuner is just below the communication radio tuner.
Modified from source
Parallel approaches (or nearby ones) will be set up so that they do not interfere with each other and an ILS must be checked periodically to make sure that any deviations are within allowances.