Try to say the following sentences as fast as you can.
1/ Which wristwatchers are Swiss wristwatches?
2/ I can think of six thin things, but I can think of six thick things too.
3/ She sells seashells on the seashore.
Pronunciation "th" should not sound like "da" or "fa".
Put your tongue between your teeth to make the "th" sound.
Look in the mirror - is your tongue just a little bit between your teeth?