Turning to create space, or Seperation turning is something you usually try to do in the middle of the field, where you want to quickly turn and face your defender with the ball so as to beat him one on one or with a pass. What you are trying to avoid is getting shut down so quickly by the defender that before or just as the ball comes, he is already forcing you towards your own goal because he is breathing down your neck. This usually is pretty uncomfortable and tends to put you in a panic as you twist and turn trying to avoid getting stripped of the ball. Basically what you have is a lack of space to move in or turn in. The answer is simple then, you must create your own space ahead of time.
First step, always be peeking around to find your defender before the ball comes, some players go so far as to try to make contact with their defender, (reach a hand out and touch them or bump shoulders lightly), so as to fix his position firmly in your mind in relation to the ball. You can't always do this as the game moves to fast, but a peek every few seconds during the building period of a play will give you all the info you need.
Second, you need to watch for the time that a pass is likely to be played to you where the defender will be chasing you, as that moment nears, you need to check his positioning closely. If he is far away, (your speed minus his speed multiplied by the speed of the ball..... err you know what I mean), and you know that you can get to the ball and turn without him getting there in time, leave him alone.
If, on the other hand, he is close, and you know that he is going to hound you the moment the pass is played, immediately start checking away and into him. Put a forearm into his chest or shoulder, keeping the contact close to your body so the ref doesn't think you're trying to forearm shiver him. Start moving him away from the ball, keeping your body turned sideways to the ball so that you can push, but still be able to move quickly to receive it when it is passed to you. The moment it is struck, the pass that is, you give one last little shove away, not like you are throwing him away but a little stronger than a mere push off, and check to the ball hard to recieve it. You should now have plenty of room to recieve, turn, and go at him. He is going to be pissed if you did it right, because he knows you shoved him, but the ref didn't call it because you never shoved him that hard or extended your arm in such a way that it looked like a foul, (By the way, never push off with both hands, that is WAAAAY to obvious to a ref). He is going to be real mad, and real intent on getting the ball back, so he is likely to dive in on the tackle, so be light on your toes and ready to cut or push away before he wacks you.
That is the art of seperation turning, or as some like to call it, Cheating with a Grin! It really isn't cheating, but it is getting away with one. What it amounts to is:
1. Recognizing a situation where the defender might have and advantage.
2. Manipulating the defender to take away that advantage.
3. Turning the tables on the defender so that now you have the advantage.
As far as I'm concerned most defenders I've ever met have not the slightest guilty conscience over doing that and worse to forwards and mids, so I can promise you I've never lost a night's sleep over the issue.
You can practice this alone just by checking away with your body turned and then checking back to the ball when it comes, but it is by far best to have two friends, one to be the passer and one to be the defender.