This is a very popular English learning channel on Youtube. Jennifer has a lovely approach, a very good teaching methodology, and a soothing, attractive voice. I’m sure her students adore her!!
Jennifer’s sentence to practise is this:
“Kathy thanked both Theo and Thelma for everything.”
Try saying the sound ‘s’. As you make this sound, push your tongue forward until it touches your front teeth. You will hear the sound change from ‘s’ to ‘th’.
Another technique which is quite effective. Put some chewing-gum in your mouth and push it on to the roof of your mouth right behind your front teeth. Now try saying ‘s’ – it will sound like ‘th’! When you can do this automatically, you won’t need the chewing-gum any more…