Flying Kick ( Instructional reference )

From on Guard stance do Step 1
Step 2
Step 3
Step 4
The Flying Kick then back to the on guard stance

Flying Side Kick
From the on guard or fighting stance take two steps forward for your approach ( or you may also take as many steps as you like before the kick ).
on your second step, plant your take off leg with a slight inverted step.
Lift your legs up together then execute the side kick. Recoil your leg back to chambered position and land softly on the fighting stance.

Note : Make sure your head is forward for balance on the landing. Keep your eyes on the target during the entire techniques focused. It is difficult to kick in the air, but setting up targets during practice will help you develop a better kick.