I find that many non-native speakers manage to set up properly for the th-sounds, but don't maintain enough tension or aren't able to move to the next sound quickly enough for fluent speech. The only difference between thin and this is the voice, i. e., the same as between d,t; g,k; f,v; s,z; etc. Nothing else changes.
Think of a snake's quickly darting tongue. In front of a mirror without adding air or voice, practice moving the tip of your tongue just between your teeth and the rapidly back again. Then add the air each time you make the darting motion. Make sure when you add voice that nothing else changes.
Finally, practice both th-sounds with all vowels and diphthongs as well as the thr-sound.