As mentioned, practice in the daylight with some unusable film. As for my $0.02 worth: I hold the reel in my left hand (film in my right) with my index finger located at the prongs. This serves two purposes: (1) it makes sure I am holding the reel in the correct direction, and (2) the left index finger is used to receive and guide the film onto the prongs. Using the index finger on my left hand, I find it easy to hook the leader onto any of the perforations I want.
As for cutting the leader off, I borrowed a technique from my hairdresser (used to be called a barber!). After removing the film from the cassette, hold the leader between your index finger and middle finger with enough of the film protruding above your fingers so as to include the narrower portion and a bit (sorry, I don't know the technical term for this part of the film). Using scissors and these fingers as a guide, cut the leader above your fingers. I'm amazed at how consistent and square my cuts have become. However, with Hewes reels the leader doesn't need to be square or between the perforations.