Say accustomed as /əˈkʌs.təmd/. Cambridge shows UK and US pronunciations identically with this transcription. Dictionary IPA uses ˈ for primary stress on the second syllable and . for syllable breaks, so the dot after ʌs marks where the syllables divide: a-KUS-tumd.
Drop your jaw for the first weak vowel, then hit KUS strongly.
Keep the final vowels weak in accustomed and hold the strong beat on KUS.
About respellings and symbols
The respelling uh-KUS-tum is a rough guide, not precise phonetics. It keeps the rhythm of accustomed but blurs the vowel detail shown by the printed transcription /əˈkʌs.təmd/.
IPA symbols stand for exact phonemes, one symbol per sound. Dictionary IPA uses ˈ to mark primary stress and . to mark syllable breaks, so /əˈkʌs.təmd/ tells you the word splits into the syllables ə, kʌs and təmd, the first vowel is weak and the stress lands on KUS.
If a respelling and the IPA disagree, trust the IPA and your ears. Respellings bend toward familiar spelling patterns, so uh-KUS-tum hides the weak final vowel and the soft d recorded in /əˈkʌs.təmd/, where the syllable break dot marks where one syllable ends and the next begins.