Basically, you want to focus on getting as much damage as possible in as short amount of time possible. Frequent ganking is a must, but don't give up desired CS for a fail gank. Take top or bot lane (hopefully you are alone) and hate spike away, trying to position the minion between you and the enemy champion so you hit him too. Don't 1v1 until you have a clear advantage. Otherwise head to the other lane and set up a gank if your lane is pushed. If you run low on mana, go into stealth for a little bit and let it speed up your mana regen until you have enough.

You will have a decent amount of health with this build, more than enough to enable you to take out the priority target in a fight or gank and escape with your W ability. You should be faster than almost all champions, and remember if the enemy has a slow, make sure you save W until after they slow you, as it will remove it.

Wait for the right moment and then bam. Use your ult, DFG, ravage, and then spam hate spike until the enemy is dead. Use dark frenzy to catch up if needed, but the passive on it should be enough to keep you within hate spike range. The reason for ultimate before dfg is because it does damage based on targets CURRENT health. DFG does MAXIMUM health (and then boosts all abilities by 20%). This also helps to make sure the enemy doesnt flash away from your ult range making you miss, and if they flash after you land it they will be slowed and you will be able to catch up no problem.

Once you get wooglets, you can be more aggressive in tower dives, as long as the target is squishy enough like an ap caster or if you run across one, an ad carry.

In teamfights, make sure you try and ult all 3 enemy champions. At the beginning of the fight. This will boost your shield to its maximum, slow all of the enemies, and make your teams ACE 15% easier to obtain. Hate spike is also aoe so make sure you position it properly to hit more than one champion if possible, but focus mostly on taking down the one who will run and then launch long range attacks from a distance. With your speed you should be able to get back into the teamfight in no time, securing the ace and then rushing to the nearest tower and destroying it.