Hi Rez,
I'm by no means the most knowledgeable player on this forum, but if you don't mind that I offer suggestions:
One of the best tips I read here was that "tonguing" is simply a matter of interrupting the air flow. In the lower register I accomplish this touching the tip of my tongue to the roof of my mouth. In the higher register, I have the tip of my tongue braced against my lower lip, so I touch the roof of my mouth with the "middle" of my tongue.
Many beginners tongue by pushing their tongue between their lips. This is more than is necessary, and disrupts your embrochure. This also gives many beginners that "blat, blat" sound when tonguing.
Don't think of it as trying to plug the sink, think of it as passing your finger through the flow of water.
Steve