For this TongueTwisting4BetterSpeech (TT4BS), we're going to practice the voiced "th". Like the unvoiced one (see Feb 11 blog), you also gently bite your tongue. However, instead of blowing, you make the sound with your vocal chords (in your throat) because it's a voiced sound.
Practice these words first:
those, these, then, their, than
bother, other, worthy, brethren, gather
rather, loathe, this, another, further
Here's the tongue twister. Remember, slowly first, emphasizing the voiced "th". Practice several times until you can say the sentence at normal speed three times.
Other than their brother, I'd rather bother those who gather worthier brethren than these.
Another way to say this sentence is: Besides their brother, I prefer to ask (for help) people who are better (nicer, richer, etc.) than the ones who are here. Yes, it sounds a little too self-righteous. Btw, brethren is an old word for brother and can sometimes mean friends, colleagues, classmates or fellow members.
Remember, practice makes perfect. Good luck!