Utterances are usually written phonetically using an alphabet of symbols
known as IPA (International Phonetic Alphabet).  This alphabet is found at
the front of most good dictionaries.  The symbols can be hard to learn and
were not readily available on computer keyboards, so the Advanced Research
Projects Agency (ARPA) came up with the ARPABET, a way of representing
each symbol using one or two upper case letters. Narrator uses an expanded
version of the ARPABET to specify phonetic sounds.

A phonetic sound, or phoneme, is a basic speech sound, a speech atom.
Working backwards: sentences can be broken into words, words into
syllables, and syllables into phonemes.  The word cat has three letters
and (coincidentally) three phonemes.  Looking at the table of phonemes we
find the three sounds that make up the word cat. They are the phonemes K,
AE, and T, written as KAET. The word cent translates as SEHNT. Notice that
both words begin with the letter c, but because they are pronounced
differently they have different phonetic spellings. These examples
introduce a very important concept of phonetic spelling: spell it like it
sounds, not like it looks.

 choosing the right vowel 
 choosing the right consonant 
 contractions and special symbols