A shadow leap is performed by tapping
and choosing where you want to leap to. You will be using this a lot throughout the the game and it should unlock naturally around chapter 3, especially if you go looking for every scroll in each chapter.
All leaps count towards the total, even if you're caught and have to reload a checkpoint so take advantage of that if you want the achievement early.
A couple of things to note if you haven't followed the tutorial, you cannot leap into bright areas and you must have shadow 'power' to perform a leap. This can be quickly obtained by standing in the darker shadows.