How can I cut paper in one cut with scissors?
Focus on keeping the paper against the point where the two scissor blades cross.  At this one point, the paper is being sliced simultaneously by both blades which decreases the thickness which each blade needs to cut through.

When the paper slides forward onto only one blade, the speed of cutting slows because one of the two blades has been removed from the cutting process.  This slows the advance of the scissors across the paper, inviting you to compensate by applying more forward pressure.  The additional pressure speeds up the cut but slams the paper into the crux of the blades, inviting a snag and tear.

Slow steady motion, with a focus on using only the crux of the blades should result in a higher percentage of success.

Aside from that,...

Practice!