Sound out the first syllable of the word, then make a list of all letters and combinations that could make that sound. For example, the first syllable of "pharmacy" sounds like far or phar. Put both syllables on the list.
Check the dictionary for all of the syllables on the list. Continue until you have found a word that appears to be correct.
Check the definition for each word. Homonyms are words that sound exactly the same, like knight and night, but have different meanings and spellings. If the definition listed does not match the word, look up the next possible spelling on the list.