The tip of the tongue touches the back of the upper teeth, or protrudes through them. Same tongue position, but the release is at the sides of the tongue. This sound is sometimes used to encourage a horse to speed up. This click is probably the loudest click-like sound you can make. Try the language Xhosa in SWOL for starters.
Most click languages are in the Khosian family.
Here is a small page devoted to Khosian languages .