The spelling of the made-up word "NPBDT" can be explained using the International Phonetic Alphabet (IPA). The first letter, "N," is pronounced as /ɛn/. The second letter, "P," is pronounced as /pi/. The third letter is "B," pronounced as /bi/. The fourth letter, "D," is pronounced as /di/. Lastly, the fifth letter, "T," is pronounced as /ti/. Therefore, the correct spelling of "NPBDT" in IPA is /ɛnpiːbiːdiːtiː/.