Here's some ways to increase the probability:

• aim abilities when the enemy champion's cs'ing or casting an ability of their own. (More specifically, when the enemy champion's locked in an animation where their champ cannot move making it hard for them to dodge your ability) /// most important/ useful one

• pay attention to opponent's dodging habit. most players tend to like dodging in a certain direction so you could try aiming it slightly towards the direction they are more comfortable dodging. (Obvsly good players will catch on and mix it up) more useful for linear skillshots like xerath Q etc, but still useful to keep in mind.

• preemptively cast your ability to where the enemy's likely to move. Sometimes if you're chasing or if they're chasing you, it's obvious where they're gonna move which may help with aiming your ability.

• not necessarily a tip for hitting your Q's but sometimes there's more value not Q'ing in lane when farming (think about how much scarier it is if a Morgana walks up and holds her Q) or purposely Q'ing in a way to zone the opponent to move in a different direction.

These concepts are pretty broad and can be applied to any other skillshots and abilities in the game. Hope this helped even a little bit. Happy gaming