When I used to teach this kick, you can break it down into a few small chunks.
1) Pivot on the front ball of the foot so your heal is facing the opponent and toes facing away. (everyone is right on )
2) Turn and look over your back shoulder to look at the bad guy, or yourself the mirror. As you do this, your back leg almost comes in a cat stance. Your weight is on the supporting leg, your kicking leg is ready to go.
3) Execute the kick(either one) through your opponents face (well wherever you can reach), and you should land as you started.
This break down really helps with balance, and takes the "spin" somewhat out of the picture until you are comfortable with turning and looking. Once you are decent with the three steps... loose step 2 so that 2 and 3 become one motion and you get some spin.. then make it all one motion with full spin.
Now for the good stuff... the difference in throwing a spin hook VS a crescent. With the crescent.. it is most helpful to stay as upright as possible and swing your leg from your opponents shoulder to shoulder (therefore going completely through the head.)
For hook kicks (which are related to side kicks in terms of how it uses your hips and your body position) you should lean back a little for counter balance and to get your hip into play. When you turn and look.... imagine throwing a side kick at the corner of the room (or the first shoulder of your opponent) and then "hook" your leg through so that you end in a round house ISH position before putting it back down. As you get better the side kick disappears and you are simply snapping that leg through their face... or leg or whatever. When you throw a hook kick.. think of being able to kick a glass off of a table so that way your kicking leg is sweeping through the horizon, and not making funny V or ^ shapes in the air.
go get em