These are made at different places in the mouth. Find the right place and the difference takes care of itself.
حHa
The middle of the throat
Halfway up the throat. ʿAyn constricts it and lets the voice through; ḥāʾ narrows the same spot with no voice, a clear hot whisper.
هHaa
The deepest throat
The very bottom of the throat, nearest the chest. The hamza closes it completely and releases; hāʾ lets the air hiss through the same place without closing.
Two words, one letter apart
Everything else in these words is identical. If you can hear which is which, you have the letter.
حَرَم
ḥaram
sanctuary
هَرَم
haram
pyramid