The goals here are to never let your weight fall onto the floor. Those shoulder blades must stay activated and squeezed towards each other the entire time you are moving. Your knees can be bent as I have shown in the video or they can be locked straight for an added intensity.
Inhale on the down
Press inner thighs together as you drawn them into your chest.
Squeeze your shoulder blades together and pull your belly button in on the up.