for th sound, try sliding your tongue between your upper and lower teeth, but don't let your tongue stick out from your teeth. The tip of your tongue should be caught in between the teeth. As you do so, exhale though them.
That should do for 'sixth'.
When 'th' is followed by other alphabets, for ex, 'this', 'thyroid', and 'parenthesis', you quickly retract your tongue from your teeth.
At least that's how I learned.
By the way, is 'the tip of your tongue' correct? or should I have omitted 'the'?
how's my explanation overall? Could I have used better structure and words?