pronunciation in English is very complicated: there are no simple rules. And it is even more complicated when you have a sound that doesn't exist in Italian: such as the sound "th" in "this" and in the verb " to think". Nevertheless there are Italian people who have a speech defect and find very difficult to pronounce the sound /s/.
When I have to teach it I usually tell my students that, if they want to pronounce it correctly, they must pretend to have this speech defect.
Then I start an activity by introducing myself pretending to have this defect myself.
"good morning, my name is Santina Di Rienzo. I was born in 1963 and I have been living in Pavia for 27 years.
"Buon giorno. Sono Santina Di Rienzo. Sono nata nel 1963, vivo a Pavia da 27 anni e sono la vostra insegnante di inglese"
Then I ask them to introduce themselves pretending to have the same defect. Practicing it is a very amusing activity for my students and they learn it immediately.
then I teach them the two following tongtwists:
the thin 33 year-old thief thinks his thights are thik
Whether the weather be fine
Whether the weather be fine,
Or whether the weather be not,
Whether the weather be cold,
Or whether the weather be hot,
We'll weather the weather,
Whatever the weather,
Whether we like it or not.