It referred to the letters by adding a vowel (usually "e", sometimes "a", or "u") before or after the consonant.
ɪt ɹᵻfˈɜːd tə ðə lˈɛɾɚz baɪ ˈædɪŋ ɐ vˈaʊəl jˈuːʒuːəli "ˈiː", sˈʌmtaɪmz "ˈeɪ", ɔːɹ "jˈuː"bᵻfˌoːɹ ɔːɹ ˈæftɚ ðə kˈɑːnsənənt.