To produce TH sound correctly you need to first stick your tongue out between your teeth and bite down slightly[θ], then blow[θ]. This created Air Friction. The sound comes from the air friction and should be pretty loud[θ] . Keep your tongue flat and maintain it even steady flow of air. I’ m not just saying stick it out a little bit, I’ m saying stick it way out at least while you’ re practising to get used to the feeling. The TH sound comes from the air and the friction. You have got to hear it and should be somewhat loud [θ], like that.