AD Marks and Quints are an obvious choice for boosting are Q damage. Armor Seals makes your laning phase more enjoying as your enemy ADC will damage you less. You can replace these with health if desired. 3 AS Glyphs are good if you want to farm slightly faster, but Varus is already a last hitting god due to his passive and his W, so this is completely playstyle based. Scaling MR Glyphs to give you enough MR when you transition into team fights against enemy AP Carries.

For our masteries we're looking to boost are Q damage as much as we can without losing too much utility. Stuff like Natural Talent instead of Vampirism will make our ability damage significantly higher, for only the lose of 2% lifesteal/spellvamp. This is a good trade off, since we're relying on our Q damage for trades and not on our Auto Attacks. At full build you can easily obtain 500-600 Attack Damage, making Deathfire Touch a very strong keystone. Fervor of Battle is more traditional, but either option is a winner.

Starting W makes rushing your level 2 much faster. Max your Q because that's our main poke, then max E second so that our Armor Pen has it's full effect.

Your passive is pretty self-explainable. Killing units will grant you extra attack speed, this is great for Varus's farm since his W applies extra magic damage. Varus has one of the easiest farming capabilities of all ADC. his extra Attack Speed will almost always guarantee that your next attack will fire quick enough to kill a minion and the W's magic damage will give you a higher success to execute a minion.

Your Q is great for farming minions, especially the weak caster ones. When poking down champions with your Q try to hit as few minions as possible to maximize your damage, also the more stacks of Blight you have applied will help your damage insanely. Your Q's range is insane making you able to snipe kills from far away and even 750+ Units from tower. The damage increasing depending on how long you charge it, as does the range. Panic Q only when needy, at other times let it charge. Use your Q as your main form of harass in lane.

W is another self-explainable skill. Extra magic damage on auto attacks is great for farming as explained earlier. W also gives a stacking passive upon auto attacks. Targets can have upwards of 3 stacks at once and will take extra damage for each stack once they have been hit with one of your other abilities. Try to apply 2-3 stacks before shooting your Q or E.

E Is great if you're in need to catch a target or you need to escape. It also helps alot if you need to slow your targets down a bit before shooting your Q. Refrain from using this ability early on and this combined with Q spam will drain your mana way to quickly.

Varus has one of the best ults for teamfights of all ADCS. His ult allows you to lock down multiple targets during a team fight allowing the rest of your team to follow through thoroughly. It is also great in laning phase if you combine it with another hefty CC ult. Examples; Morgana, Braum, Sona, Nami, Etc. Lead with this or use this when escaping targets.

If you're a traditional ADC than Heal and Flash are a no-brainer.
If you're against a heavy CC team, then you might want to take cleanse.
And if you want a more aggressive approach, ghost is definitely the way to go!

Against some champions you may want to stay back further from minions (Main examples being Sivir, Tristana, and Lucian) Varus's farm is easy with his Attack Speed boosting passive and his extra Magic Damage from his W. Freeze lane and push when you need to, give the jungler some space to gank. Farm with Autos mainly and Q if you want, but don't spam it due to the lack of mana sustain.

Pros:

Massive Q Damage from a safe distance.
One of the easiest to farm with ADCs.
Ultimate is great for teamfight compositions!
Synergizes well with heavy CC comps.
Harass' fairly well with his Q and E.
Can be surprisingly bursty if ahead.
Some of the best skins you'll ever see on a champion.

Cons:

Lacks Mobility; limited to CC.
Struggles against most META champions.
Easier champion to all in, or catch out.
Harass can be quite predictable.
Can be mana-starved without Essence Reaver.
Doesn't feel like most ADCs, very unique caster playstyle.

The absolute best supports you can have for Varus, are ones that can peel or ones that can chain CC.

My personal Favorites;

Leona; Very strong initiation, lots of peel, multiple CC abilities, strong passive for landing Varus Qs.

Taric; Good CC, Decent peel, tons of survivability, Ultimate allows for Varus to stack up W safely before Using Q.

Sona; No peel, Fair sustain, Good ultimate for CC chaining, Sona's Q and Varus Q apply some serious burst.

Other Strong Supports

Morgana
Thresh
Nautilus
Janna