Step 1: Palm squeeze and light
Take your Zippo-ing hand, pinch the top corner just above the hinge in between your index and thumb, hinge facing up.
Now your middle finger should push on the top corner of the lighter on the opposite side from the hinge, if you're having trouble take a look at the photos, to make sure you have it right because it's not that easy to explain.
Now push the lighter in with your middle finger allowing it to catch on the webbing of your fingers, with this one pressure isn't the key, it's more about getting a continuing motion.
Now bring your middle finger round and light it.
Flick your wrist back to close the lighter.