Practice, lots of practice: I have found, for me, that Mexican Ping leaf cuttings rot if they are "too wet" and shrivel up if they are "too dry". If the moisture, humidity, temperature, and light are also at the right level, then you will soon see small plantlets form on the basal end of the severed leaves.
Sometimes if the moisture level is just a little more than optimum you will get small plants to form quickly while the parent leaf quickly rots away.