What is the key difference in how the /ð/ and /θ/ sounds are produced?

The /ð/ sound is voiced, using the vocal cords, while the /θ/ sound is unvoiced, produced by air pushing through the mouth.

Explain the tongue placement required to correctly pronounce the /θ/ and /ð/ sounds.

The tongue should be pushed slightly through the teeth, with a bit of tension, allowing air to pass over it.

I can tell the difference between the /ð/ and /θ/ sounds when I hear them and say them.

1
The two 'th' sounds in English are voiced and unvoiced, and they require the same mouth position.
2
Content words usually begin with the unvoiced 'th' sound, while structure words often begin with the voiced 'th' sound.
3
When the letters 'th' appear in the middle of a word between two vowel sounds, it is usually the voiced 'th' sound.