Varus is a AD carry so you just need to farm use some combos and other stuff.So i get a lot of kills with my Q spell it has a very big range and when they run from you, you can get the kill very easy.I use the E spell to slow them down if i attack them or if i run from them so use it a lot.You take the W spell second because its a very good combo 3 basic attack and then the Q spell or the E spell.In team fights you don't need to go first because you will get killed just attack from range and pick up the kills.Don't forget you are the carry so in bot lane with the support you get the kills and the farm.Your ultimate can stun the whole team if they are very close to each other so all your spells have very big range and in early game and in late game they deal massive damage.My scores with Varus aren't like 15/5 or something but never mind my scores are more like 8/0 i care a lot not to die sometimes i go for the kill but play safe because you are very squishy.At early-mid game i keep the Doran's Swords because i have more health that way and a little damage then i sell 1 and by a Vampiric Scepter but you will see that with 3 Doran's Swords and the mastery's for life steal you will have 12% Life steal more then a Vampiric Scepter.In the lane don't push just take last shots so the jungler can gank also for the summoner spells you take flash because is the best summoner spell in the game and you can get out of ugly situations very easy you can jump walls and other stuff and you use heal because in early game it's very useful to stay in lane more and other stuff.
+ Ranged AD Carry - Squishy!!!
+ Good CC! - Need farm to be useful
+ Multiple Root - No escape skills
+ Awesome Farmer
+ Awesome Harras
Also i forgot to say about the passive that whenever you kill something you get attack speed if you kill a champion your bow gets white and you get 40%Attack speed and if you kill a minion you get less you get 20%Attack speed and your bow is red.Your runes are for early-mid game to be more defensible.So about the Quicksilver Sash and the Guardian Angel you will take Quicksilver Sash if you have in the enemy team long duration CC.And about the Guardian Angel you take it if you have a good team that cares about the carry and if you die and revive to not die again.So with the mastery's with the offensive mastery's you will be stronger in early game with the crit and attack speed and the life steal with the 3 Doran's Swords and with the defensive mastery's in early game you have more Health and Magic resist.
Here is a more details about your spells.

Living Vengeance (Passive):
"On champion kill or assist, Varus gains 40% attack speed for 6 seconds.
On minion kill, Varus gains 20% attack speed for 3 seconds."

Piercing Arrow (Q):
"Varus starts drawing back his next shot, gradually increasing its range and damage, then he fires it, dealing minimum 10 / 43 / 77 / 110 / 143 (+1.0 per attack damage) to maximum 15 / 65 / 115 / 165 / 215 (+1.6 per attack damage) physical damage, reduced by 15% per enemy hit (minimum 33%).

While preparing to shoot, Varus cannot autoattack or use abilities, and his movement speed is slowed by 20%. After 4 seconds, Piercing Arrow fails but refunds half its mana cost. "

Blighted Quiver (W):
"Varus' basic attacks deal bonus 10 / 14 / 18 / 22 / 26 (+0.25 per ability power) magic damage and apply Blight for 6 seconds (stacks 3 times).
Varus' other abilities detonate Blight, dealing magic damage equal to 2 / 2.75 / 3.5 / 4.25 / 5% (+0.01% per ability power) of the target's maximum health per stack.

Hail of Arrows (E):
"Varus fires a hail of arrows that deal 65 / 105 / 145 / 185 / 225 (+0.6 per bonus attack damage) physical damage and desecrate the ground. Desecrated ground slows enemies' Movement Speed by 25 / 30 / 35 / 40 / 45% and reduces their healing and regeneration by 50%."

Chain of Corruption (R):
"Varus flings out a tendril of corruption that deals 150 / 250 / 350 (+1.0 per ability power) magic damage and immobilizes the first enemy champion hit for 2 seconds.
The corruption then spreads towards nearby uninfected enemy champions, applying the same damage and immobilize if it reaches them. The corruption will continue to spread until there are no further unaffected targets in range."
Okay so you are going to be targeted it team fights, deal with it!
try to stay in back and target their AD Carry/the most fed champion.

Use your ultimate on the correct time, not too early and not too late.

Focus the enemy's most fed squishy champion, take him down and than move to the next one in the queue, like team can clear all the other team easily without dying.
So this is not my first guide i have a guide about Olaf and Gragas check them they are called "Ultimate AP Gragas" and "Legendary Olaf" give me your opinion in the comment box and everything that you think its wrong you can just write it and to tell me why its wrong.
So see you in my next guides i hope you will like them all.