Practice these words first:
unvoiced "th": Samantha, thorough, thesis, thought, thinking, threatened, throw, thesaurus, path, thawed
voiced "th": Although, with, their, they, loathed, the, rather, themselves, worthy, this
Now for the tongue twister. Remember, slowly first emphasizing the sounds; then repeat until you can say it at normal speed three times.
Although Samantha was just thorough with their thesis, they thought she loathed their thinking and threatened to throw her thesaurus on the path before it thawed.
To be thorough means to go through everything without missing anything. For example, when you're cleaning your room or checking your essay for mistakes, you want to be thorough so that your room is super clean or your essay is perfect when you are finished.
To loathe means to hate intensely.
Remember, practice makes perfect. Good luck!