Here's my typical game order for Veigar mid, and I've been playing him as my main for a couple years now:
Pre-Reqs
- Take Flash and TP for Summoner Spells
- For runes, get MPEN reds then a mix of mana regen/AP/CDR
1. Start Mid-Lane with Doran's Ring (if you don't get mid lane, he isn't worth playing because of lack of Q farm although top might not be a bad option, but you usually don't have 2 ap lanes on a team)
2. After Q farming up to 740g while dodging enemy mid, recall back and grab lvl 1 boots and meki pendant to enhance movement speed and start working on your DFG. THIS IS THE MOST IMPORTANT PART OF VEIGAR. The meki pendant should help with your mana regen to farm your Q better. TP back to mid without missing much action.
3. At this time, you should continue to farm your Q and by the time you hit level 4, you should be able to harass the enemy mid with stun (E) and dark matter (W). Don't overdo it though because you still need to save mana for Q farm.
4. Once you hit 6, gauge your enemy's hp and you might be able to kill them with a stun E-W-R-Q combo. Don't force it though if it's not there, you will be nuking them soon enough.
5. All the while when you're farming up your Q, recall back to add on to your DFG as needed. If your lane is pushed to the enemy, don't be afraid to go back to the shop then run back to lane if TP is done. You won't miss out on too much action. The fiendish codex portion of your DFG makes it really easy to farm your Q beacuse of the mana regen and CDR.
6. Once the magical moment happens when you have enough money to finish your DFG, that's where all the fun happens. Now you have a crazy ass nuke combo that will erase most enemy ap champs off of the map. Simply sneak up to the bushes while they're pushing mid and E-W-1-R-Q combo them to death. It might take some practice getting down, but once you do, it's a great feeling. (Note: When doing the combo, your W will be clicked first but your 1 should be hitting first for max dmg based off %hp while your W is still falling. Also, DFG should be placed in the "1" slot of your inventory for ease of use)
7. You should be nuking someone everytime your cooldowns are ready for maximum efficiency. Other items to start building on are the following:
-Rabadons
-Void Staff (if you're doing well, this will allow you to even start 1 shot nuking tanks!)
-etc.
Keep farming your Q throughout the game and your AP will grow tremendously. Very important to do this when able.
FYI - Whenever people see a veigar, they do one of two things:
1. Laugh at the noob who doesn't know how to play him
2. Sh*t their pants because he's a vet who knows his ****
(Be #2!)