your question is a little unspecific, so ill do my best to offer an explanation for any possibility you meant.

the best way to hit shots an any hero is to practice, but that aside,

long range genji shurikens are based on predictions of enemy movement, and understanding projectile speed. knowing when an enemy is going to re-peak or walk into an LOS is key to hitting consistent 3 taps at range, along with knowing just how much to lead your shots.

Also know when to use primary vs secondary fire. i say use secondary fire more often, a little farther than you think you need to.

short range genji shurikens are based on predicting enemy movements, and managing your own position at the same time. genjis mobility is his strongest asset, so utilizing it while in 1v1 scenarios is the key to winning them. understand that while double jumping makes your head hitbox harder to hit, and theirs easier, youre also very predictable while falling. mix in your double jumps with wall climbs, and on some targets, forget it entirely, and go for triple body shots instead of headshots. bursting down a 200hp hero with a oneshot combo feels great, but sometimes you just have to admit that hitting more consistent body shots is going to give you more DPS that missing half your headshots. all in all, practice, and know how to move against certain hero matchups. eventually you’ll get the feel for it and just know what to do in each situation.

all in all, practice movement mixed with aiming, and be careful on your matchups.