These two sounds, D and TH, differ in location—where they’re made, and in manner—how they’re made.
/d/ is created by pressing the blade (front top part) of the tongue up firmly against the roof of your mouth, behind your upper teeth where the alveolar bumps are. Then push away as you say the sound. The airflow is first stopped by the pressure, and then resumes when you release the tongue and make the sound./d/ is a “stop” consonant, called so because it stops the airflow momentarily.
/ð/ is created by touching the tip of the tongue to the back of your upper teeth. As you say the sound, allow a continuous air stream to flow around the sides of the tongue and out of your mouth as you say the sound.
Both sounds are voiced—they are created with vocal fold vibration in the throat.
If you’d like to see and hear more about the sounds of English–how they’re the same, and how they’re different—check out my YouTube channel