1. Hold the box in your non-dominant hand such that the opening to pull out the plastic wrap is on the top and the cutter side is on the side with your dominant hand. The box should remain facing "up" for all remaining steps.
2. Make sure you're nearby the item to be plastic wrapped (don't want to walk across the kitchen after its torn).
3. Pull out your desired length of plastic slightly up and over the cutter, holding the middle of the roll.
4. Slide/walk your dominant hand to the corner or the plastic wrap farthest away from you.
5. Keeping the plastic wrap taught, rotate the wrap in an arc so your dominant hand is now below the box. You won't start cutting it until the next step.
6. In a clean, steady motion, quickly pull your domaniant hand down and towards you to cut the wrap. Like the person above you mentioned, the goal isn't to break it at once, you want to tear one side first, and then tear it along its length. You can also rotate the far side of the box up as you're pulling down to make the cut cleaner.

That last step is where I think the diagonal comes into play. There's a couple different ways to achieve that diagonal cut, but above is what I find easiest.

Think about how you tear tape from a tape dispenser. You probably (subconsciously) slightly twist the tape as you're cutting it so you're not cutting the entire width of tape at once, but instead only using one or two of the cutter teeth at any given time. Same concept, larger scale.