These are made at different places in the mouth. Find the right place and the difference takes care of itself.
دDal
The tongue tip and tooth roots
The tip presses against the roots of the upper front teeth — where the tooth enters the gum — and cuts the air off. Ṭāʾ, dāl and tāʾ are all made exactly here.
ضDad
The edge of the tongue
The side of the tongue presses against the upper molars — usually the left side — and the sound rolls along it. This is ḍād, the letter Arabic is named after.
Two words, one letter apart
Everything else in these words is identical. If you can hear which is which, you have the letter.
دَلَّ
dalla
he showed the way
ضَلَّ
ḍalla
he went astray