Basically during early game you'll just stay far from enemies and try to poke them with W in the wave to stack passive and push the wave.

+25 stacks : You can stack farming by autoing minions and last hitting 3-3 melee and caster minions to rush your 125 and 225.
Should reach 25 stacks around 4-5 minutes

+125 stacks : you become a threatening poke champ with high range and good damage since you should have 2 full items. Stacking becomes faster at this point.
Should reach 125 stacks around 12-13 minutes

+225 stacks : Once you reach this threshold, the game is basically free if your team plays around you (you should reach this amount of stacks around 20-23 minutes if having a normal game)
At this point you're an extreme threat with 6% max health damage every Q with an execute on this same spell. Keep away from teamfights and don't get caught, you're the win condition of the game.
Should reach 225 stacks around 18-20 minutes

You could think that AFK farming and poking is the fastest way to stack but do not forget to follow your team during fights, hitting an enemy champion with an ability give a stack, with some Qs and Ws in a teamfight can easily get you 20 stacks early on.