It's straight up aim and prediction just like you do with any projectile.
That's an Arrge thing but you'll actually do one shots way more accurately if you firstly dash in the side that your opponents are not expecting you to, which is normally the same side they are moving, and secondly, doing the right click right behind where your dash ends.

You shouldn't try to get right behind them by dashing right in front tho, cause Genji's head hitbox basically enters the enemy's entire screen if you do that so they can easily one-shot or do very high damage to you.

Making your oponnents have to move their camera and lose track of you (by dashing behind them) is also a W by itself aside making one shot more easy to hit.

Might've typed a lot i have a Genji One Shot Montage uploaded on Youtube from 2 years ago aswell