This build is made for an ADC, focus's on dealing tones of damage with your basic attack but has low survivability.

Passive:
Living Vengeance - Good for gathering CS/Great tool for multi-kills
Piercing Arrow - Good poke/high damage all game/decent mana cost/low cooldown/decent splash damage
Blighted Quiver - Add's magic damage to basic attack/good early game/useless late game
Hail of Arrows - Decent escape method/slows enemies from getting away/decent cc
Chain of Corruption - Good for closing in on kills/gets multi-kills in laning phase/great in team fights/great crowd controll/great escape method/decent damage

Doran's Blade - great early game item/good price/useless late game
berserker's geaves - movement speed to get around/decent attack speed
Phantom Dancer - great attack speed/good movement speed/decent crit chance
Infinity Edge - great damage/decent crit chance/great crit damage
Bloodthirster - good damage/great lifesteal/great damage with stacks
madred's bloodrazer - great for killing tanks/good attack speed/good damage/decent armour
Guardian Angel - decent armour & magic resist/great survivability passive

Greater Quintessence of Attack Damage -Good early game damage
Greater Seal of Evasion - Give good full game survivability
Greater mark of desolation - Give good full game armour penetration
Greater glyph of replenishment - Allows you to poke more often early game

Early game when farming stand near enemy minions well holding the S key on you're keyboard so you do not auto attack the minions and when the minions are one hitable right click them then hold S again and repeat.

Early game when farming make sure to use your Piercing Arrow to harrase the enemies. When engaging for the kill allways use Hail of Arrows behind the enemy to prevent escape.

Late game you will be dishing out tones of damage in team fights, make sure to stand at the back because you will most likely be the squishyest player on you're team. Also remember when you're build is finished you can switch out your berserker's geaves for Boots of Swiftness or you're Phantom Dancer for a Trinity Force for extra movement speed since you're attack speed will be high enough.

In one versus ones allways make sure to to place Hail of Arrows behind the enemy after you use Chain of Corruptionand make sure to use Piercing Arrow as often as possible.

After taking out the first turret and you push you're lane as fare as possible go give the other lanes a hand and end the laning faze.

In team fights, you will be one of the highest damage dealers on your team, so you will need to sit at the back and take the least damage possible. Try to wait for your tank to enter the battle before you. Also remember to focus the squishyest players on the enemy team first, Enjoy your Multi-Kills.

Yes, Varus is verry underated and is used verry rarely used in ranked matches. What people don't know is that Varus is a VERRY high skill caped champion and his abilities can be much more effective if the player has profected them. In teamfights you can cleanup multi-kills and leave with nearly full health.
Well, this is all I have to teach you about Varus I hope it helped and or influenced you to give him a try.