This PPT is designed to give your students some techniques that will get them used to the tongue placement neccesary to pronounce TH sounds correctly. The KEY to this lesson is students must first use their finger to feel if their tongue is protruding from between their teeth. Once you've practiced saying words like that, provide your students with a lollipop that they'll hold in front of their lips. When they say a TH sound correctly, they'll be able to taste the lollipop and the positive reward will really help them remember the tongue placement.
I combined this PPT with two YouTube videos by The Accent's Way. She has clear pronunciation of voiced and unvoiced TH words.