Hold down whatever your sneak button is to go down.
Essentially flying is like this (I'm doing this in generalisations as you may have altered your settings)
Jump button (default is space) twice to activate fly mode, hold it in to gain altitude
Movement keys (default WASD) to move about in the sky, like you would on the ground
Sneak button (default is..shift I think?) to descend. Once you hit a solid block fly mode will be disabled
Alternatively to go down at fall speed, just double tap your jump button again to deactivate flight mode...and plummet