My way...
Unroll the backing paper until you get to the film then tear off the backing paper about an inch or so behind the leading edge of the film. You now have the reel of film (still wound up) with backing paper around, it so that you can safely hold it, with about an inch of bare film sicking out. By having the film still on the reel it helps to keep the leading edge of the film flat. While holding the rolled up film in one hand, and using a finger under the leading edge of film to keep it sticking out (can't explain it any better), have the spiral in the other hand, resting on a table and with your finger and thumb holding the edge of the spiral right where the film needs to go in. Bring the two together and using the fingers holding the spiral to feel where the film is, start the film into the slot. Push it as far as the balls and then you can unwind the film and carry on.
Actually, that's not exactly what I do. I've posted this before but I cut two notches in my spirals so I could feed the film in after the balls. So, I start the film through the notches then just keep unrolling the film bit by bit from one hand and just continue pushing it into the spiral.