The “th” sound is made with the tip of your tongue between your upper and lower teeth, and with moving air. If you speak with an accent that tends to substitute an “s” or “z” sound, such as Russian, think of the “th” like an “s” with the tongue between the teeth. If you speak with an accent that tends to substitute a “t” or “d” sound, such as an Indian dialect, allow space between your tongue and teeth and say the sound gently so you don’t stop the air with your tongue.