IPA: /ˈhiːd.fəl.i/
KK: /hid-fəl-ē/
In a way that shows careful attention or consideration.
She listened heedfully to the instructions before starting the project.
Heedfully → It is formed from "heed" (from Old English *hēdan*, meaning to take care or pay attention) and "-fully" (meaning in a manner full of). The word "heedfully" means in a manner that is full of care or attention.
Think of 'taking care' ('heed') and doing it 'fully' — that's why heedfully means to do something with full attention.