I can't speak for the Iwiss crimper in particular as I have a cheaper one. I can tell you that it takes practice and in a given sitting I screw up about 20% of my crimps anyway. I don't know what youve learned so far, but maybe this can help:
a) Try to strip your wire as accurately as possible. If you leave too much conductor showing, it will end up not allowing the male pin to insert deeply enough when you put your connectors together.
b) Determine which way the pin goes into the crimper. The conductor half crimps smaller than the insulation half and you can see the size difference when the crimper is fully squeezed.
c) Place the wire into the strippers notch first, lining up the end of the insulation at the demarcation point between the insulation and conductor crimps. Hold the open crimper with wire aligned in place in your left hand.
d) With your right hand, place the pin into the crimper notch sideways, open side first. Orient it as determined in step b. Of course, the crimpable sections have to be roughly within the confines of the notch. The pin should stay in place by friction and compression forces on it from the notch.
e) Use your right hand to squeeze the crimper handles together completing the task. While doing so, maintain your control of the wire location in the notch with your left hand.
Well, that's how I do it, anyhow. Good luck and let us know how it goes.
-Ron