So glad I linked to an explanation....
Did you read any of it?
They can choose from certain sets of approved kanji.
Many people choose based on visual balance between characters, the meanings, the sounds, what is popular at the moment, etc.
Some people consult fortune tellers or the like and choose based on the number of strokes in the characters (shades of Numerology at work here). I remember when my son was in kindergarten here I accompanied him on a trip to Disneyland by bus. During the ride the tour guide helped kill time by passing a microphone among the parents and having them explain how they chose their child's name and how it was written. I was appalled to hear probably about 2/3 of the parents had gone the fortune teller and numerology route. The rest were things like characters from a favorite television show, a popular actor/singer, or whatever happened to be trendy at the moment.