Veigar is all about his Baleful Strike (Q)! no matter what you do constantly and through out the whole game farm it as much as POSSIBLE!!!

Pretty close to level 6 is when you should start poking especially if youve gotten your chalice when you think you are ready to begin poking your opponent begin with landing your stun and when it has stunned them drop a dark matter on them do this a few times before you throw your ult and try to finish them off with a Q.

the order the skills should be use to get a kill should be.
start with a Event Horizon leading directly into a Dark Matter Before the DM lands through your Death Fire Grasp at them (if youve gotten it) After the DM lands throw your ult in and then finally use a Baleful Strike if they are still alive!
That should kill just about anyone you are playing against as long as you have constantly Farmed your Baleful Strike

When farming try not to ever use your Dark Matter to farm minions...
people make this mistake all the time and therefor do not get Baleful Strike as Farmed as it could be
Always try and finish minions and kills with your Baleful Strike
of course as an exception if minions are hitting your tower drop a Dark Matter on them to keep towers healthy

Take flash for either escaping or chasing down an almost dead foe.
take clarity for the mana, due to the fact you will constantly be using abilities. (also if you are out of mana stick around lane not using clarity right away you might be able to bait in a greedy enemy that thinks you are defensless without mana.)