The spelling of the word "bill coo" can be explained using the International Phonetic Alphabet (IPA). The first part of the word, "bill," is pronounced as /bɪl/, with the "i" sound pronounced like in "ill." The second part of the word, "coo," is pronounced as /kuː/, with the "oo" sound pronounced like in "moon." Together, the word "bill coo" is pronounced as /bɪl kuː/. This phonetic representation helps to accurately convey the pronunciation of the word and is useful for language learners and linguists.