I was going to write this section, but I found this video on the Youtube.com.
It describes quite well how to use a blue ball marker.
Be aware that these small ball markers can be fiddly to use, and so some referees will use a larger ball marker made of Lucite or Aluminium to mark a ball for cleaning. Being of a size larger than a cue ball, these ball cannot, of course, be used to determine a free ball or whether a ball will fit on its spot.
It describes quite well how to use a blue ball marker.
Be aware that these small ball markers can be fiddly to use, and so some referees will use a larger ball marker made of Lucite or Aluminium to mark a ball for cleaning. Being of a size larger than a cue ball, these ball cannot, of course, be used to determine a free ball or whether a ball will fit on its spot.