The glissade belongs to the family of linking steps and is one of the most important, but also one of the most difficult to execute correctly. by Robyn White Artistic Associate Queensland Ballet

Here is a glissade derrière as used in allegro, taken in slow motion.

One:  You must start your glissade from a demi plie in 5th position with the weight firmly on both legs.  Too often, dancers try to start a glissade with their weight more on one leg than the other.
Two:  Before you do any transfer of weight, extend the first leg to second position with the weight firmly on the ball of the supporting foot, ready to push off. 
Three:  Now you push up, not so much out for an allegro glissade, until both your legs are stretched in a second position just off the ground, both feet stretched.
Four:  You land on your first leg and your second leg must stretch in second position, just off the floor, before you close.  Be careful not to close the second leg before you do this!
Five:  Now you close the second leg into 5th position again, making sure your weight goes onto both legs.  Now you are in an ideal, centred position for the next step!