Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Try to hit as many minions in lane as you can, wait for them to all get low, then kill them all with your E. Try to optimize mana use early game. Middle and Late game, you will be really helpful in team fights. Use your Q to sneak behind the tanks and fighters to the squishy ADCs and assassins. Land your W and then get max stacks of poison off to prime them for your E. As they run away use your R to secure the kill, which will reset your Q cd, and then run away from the enemy who just barely realized that you are there. It's pretty simple once you get the hang of it.