Select a pre-established phonetic alphabet. The UCL (University College London) website has a listing of standard English phonetic symbols, while Omniglot guide to alphabets and languages has a list for the less common International Phonetic Alphabet.
Divide a word into syllables. This will help you isolate the sounds. You might wish to separate the syllables using a dash.
Convert the sounds in the word into phonetic sounds. Many phonetic symbols are different than the letters of the alphabet, so you might need to practice writing them.
Place a line over stressed syllables in words. This helps a reader know which syllable should be emphasized when pronouncing it.
Use a dictionary such as Merriam-Webster, if you have doubts about spelling. Most dictionaries have phonetic spellings for every word.
Use an online tool such as the IPA typewriter to write phonetic spellings for word processors or online documents.