For same-handed scissors, your thumb goes in the handle for the blade that is further from your palm, while for opposite-handed scissors your thumb goes in the handle for the blade that is closer to your palm.

The ergonomic assumption is that you'll use your fingers to lock one blade in place, then use the ball of your thumb to apply a pressure away from your palm to the handle of the other blade. Because of the hinge in the middle of the scissors, this pushes the thumb's blade towards your palm as you close the scissors.

Thus for same-handed scissors the blades are pushed together, while for opposite-handed scissors they are pushed apart. Being pushed apart can form a gap that paper can fold into, or can combine with the resistance of the paper to make a gap that the paper folds into as you start cutting.

To use opposite-handed scissors, push the blades together. For example by closing the thumb blade using pressure from the pad of the thumb instead of the ball while keeping the finger blade fixed. Or, my favorite, using my thumb to keep the thumb blade fixed while closing my fingers and applying pressure away from my palm with them.

Double upvote!!!! I just found this out a few weeks ago. Over 59 years on this planet and never knew scissors were specific to one hand or the other. I'm a bit ambidextrous and tried using right handed scissors in my left hand for the first time recently and was confused why they weren't cutting. Lo and behold they magically started working after I switched to my right hand!

Funny thing despite being left handed I prefer right handed scissors :3 I used left handed ones for a year in primary cause my mom got me some, but I hated them so much I went back to the right handed ones next year