This is perhaps the simplest way of making a click sound although it's not the strongest or most realistic of the click sounds.

Standard Beatbox Notation (SBN)

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1. Position your tongue on the roof of your mouth and close your airway (so you can only breathe through your nose).
2. Try and breathe in slightly to increase the pressure above and behind your tongue.
3. Release your tongue and the release of suction makes a click sound.