Pros:

True Damage passive = delete anything in channel with full stacks
Multiple Disengage ability with E
Assassination of Priority targets with R
Safe FARMING and good late game
Plays OK from behind due to true damage potential
Outplay potential with zhonyas and E
Cons:
0 mobility vulnerable to gap closers
E has short range for all in's
Velkoz Q is hard to land and takes practice
Vulnerable during R
Requires good positioning in fights

Farming with Vel'koz is quite easy, his true damage allows him to get alot of creeps with only using two W's. What i would recommend if they have a champ that can contest your push. You W x2 the first wave and then last hit the creeps. E W Q is the standard full wave clear/push technique use the Q at an angle where it gets multiple minions at once. and aim your E towards the larger portion of the creep wave. As velkoz you can also take jungle champs lvl 6 after obtaining a kill on the enemy champ and if the enemy jungler isn't nowhere to be found close to you, you should take his raptors for easy free cs if you happen to know the whereabouts of their jungle. Place control wards in the bush next to the bush near the raptors.

You rush lost chapter for the mana regen then you build luden echo for the 500 mana and ap it grants you. Then sorcerer boots for flat magic pen and then after that oblivion orb for more flat magic pen. This will help you delete any non tank targets during the mid game ALOT. After the team stacks HP or MR you build liandries for the %hp damage. The reason we don't build void staff is because your true damage passive is enough for the enemy team to suffer. Only build void staff when an enemy stacks 2 or more MR items. You can build rabadons anywhere after oblivion orb depending on how ahead you are as the rabadon grants you MASSIVE amount of ability power with works really well with your already great AP ratios.

Velkoz has a variety of combos that can be used. His passive is like a mini game. Land 3 spells on an enemy and they get true damage from your passive.

Normal Combo on enemy champ
Q to slow them down ----> approach enemy target ----> E W {q| R
The Q there is optional and is only required when available. This combo is your bread and butter for deleting any target.
Play Video
Advanced Velkoz Combo (teamfights)
Land a Q on enemy squishy
wait till your tanks/front line engages hit their frontliners with E W Q R
Prioritize the Ray on the squishy target
All in'd by enemy assassin
ZHONYAS before the burst of damage hits you then depending on the assassin land a E where you predict they will attempt to dodge E W Q Flash max distance away from assassin then R
Use barrier and Q W again after if they are not dead or have evaded your R channel for a kill.
Safe Channel Combo
Land a Q on one of the enemies chasing you. E,W where the enemies are. Flash behind you for safe distance and use Ultimate/R
Play Video

Vel'koz has a unique reaction with yasuo so if u have a yasuo top or mid set them up for plays such as this one
Play Video Vel'koz works well with tanks that can engage for you so you could easily fry a team with E W Q R i.e leona, alistar, ornn, malphite
Vel'koz does well with a teamfighting comp due to his poke and massive AOE damage.

Ranked oh boy. one that i haven't even mastered to this point. Speaking on a vel'koz perspective your ultimate goal is to win your lane and help your bot/top lane or shove an enemy then roam bot or top for a kill. You can even roam to gank with your jungle if they are competent enough to know whether or not they could engage on the enemy jungler. Play Video This is an example of a velkoz counter roam when the enemy yasuo went bot. The power of his to change a bad fight into a winning fight.