For the correct pronunciation of /θ/ (like in think) and /ð/ (like in this) the tip of the tongue should lie between the teeth. /θ/ is a voiceless sound and /ð/ is voiced. If you mix these sounds up with /s/ or /z/, there is a chance of saying the wrong word.
sank /s/ — thank /θ/
sing /s/ — thing /θ/
zen /s/ — then /ð/
Confirm your action
You can continue this exercise later on from the point where you stopped.