I used to just cut the top off of the box using a kitchen knife. I would start just below one of the corners, with the box on it's side, and I sawed through it.
I got some funny looks for that one when I first did it on autopilot around other people lmao.
You know that the serrated lines on the box aren't great when cutting the cardboard with a kitchen knife is legitimately easier. The blade wasn't even serrated.
See... what you're supposed to do... is... hold it between you're two hands. Kinda lie you're praying. jab your two thumbs into the tab, then pull outward and just rip the box in half. Alternatively, if you're the Hulk or something, you can just grab either end and rip it in half that way.