I used pruners until I figured out how to do it with a knife. Since you're putting the edge up against bone - the Atlas joint it pretty tough - don't use a delicate blade, think Mora instead of Havalon.
I can't find the clip online, but Farming the Wild did an episode on field processing. Grab an ear and pull to tension the neck. Put the flat of your knife against the back of the ear and slide it down untol it hits skull, where it will slide into the Atlas joint.
shows a similar method. Dude uses a fillet knife.