I'm going to try to be as helpful as I can so please do not think I am trying to insult you or put you down. I want to try to help you, as you seem like you are probably not level 30 yet and are new to making guides.

Your itemization is quite questionable. I understand the concept behind Nashor's Tooth and it is indeed an underrated item HOWEVER, Varus does not have room for this type of optional side item in a solid AD Carry build. You claim the CDR is to get your abilities off cool down faster, but your Auto Attacks will do much more damage than your abilities do. Therefore, you want to make sure you are maximizing your right click damage as your first priority. Infinity Edge has less flat AD than Bloodthirster. The reason it does more damage is because of the crit chance and the bonus crit damage. However, you need to provide supplemental crit chance in your build to make this optimal. This is why Phantom Dancer is much more essential than Nashor's Tooth. Phantom Dancer also gives you bonus movement speed which is essential to maximizing your potential with Varus since he has no repositioning tool, escape mechanic, or mobility steroid. Your overall average DPS is much higher with Phantom Dancer than it is with Nashor's Tooth.

You should also NEVER start with a Doran's Blade. Your claim about it giving you more survivability is actually false. Doran's Blade gives you 80 bonus starting health, Health Potions give you sustained 450 health bonus health worth of healing. Starting boots + 3 pots also gives you much more movement speed to avoid ganks and skill shots.

Your references about lane compositions seem uneducated. You talk about laning with bruisers and tanks, not supports. I think you need a lot more experience on actual strategic play style.

Your build is not complete and you do not show what item to buy as a 6th item for late game, nor do you have a situational itemization section. Last Whisper ends up being much stronger than Black Cleaver late game unless no one on the enemy team buys armor items.

As far as skill sequence goes with Varus, it is best to actually max Blighted Quiver last. The bonus damage on hit and the detonate are both good to have, but the subsequent points in Blighted Quiver do not increase your damage output as much as putting points into your other 2 abilities would. You can start with whatever ability you prefer at level 1, but you should max Piercing Arrow First, Hail of Arrows second, and Blighted Quiver last. Obviously put points into your Ultimate whenever it is available.

You also immensely under value your ultimate it appears. No it is not a stun but the snare is very useful and the fact that it can spread is a bonus. It also is a 3rd ability to detonate Blight stacks with. It can help you peel for yourself in team fights, as you do not have high mobility or escape mechanics.

Your masteries are honestly just really bad. It probably comes from a lack of experience but if you do the math for the points you'll see it to be beneficial to alter these points. Havoc is a very low efficiency section of the offense tree. You buy IE but don't even have the point to increase your crit damage. I suggest putting your spare points that you have in the utility tree and in minion damage reduction into flat bonus armor and mr.

Your guide by itself regardless of build, masteries, runes, etc. needs a lot of work as well. You need more coding, more in depth sections, more actual game knowledge included, math to support your theories, etc.

I hope you do not take my down vote personally, as I would love to see you make a new guide from scratch with improvements for me to upvote buth with the current state of this guide, I feel the need to down vote. I apologize.

Unlike Fancyz comment I will actually try to help you with your items.
You have the right idea on going for attack speed but the item choices are incorrect. You should build Zeal and Zeal into Phantom Dancer that will give you more crict chances, and better attack speed. You also do not need to build Dagger your Boots are well enough to get you the Berserker's Greaves. You also need more pictures on your sections. ALSO you need more sections on. Runes, masteries, item explanation,team fights, summoner spells. Plus take our Clarity, you do not need Clarity, replace Clarity with Ignite it's much better on an adc like Varus

I think you have A LOT of work before this build is even close to being useable. Your items are not completed to a full late game build, your skill selection is not optimal, your guide has no coding, you have no real information that makes it more useful than any other Varus guide, and there is no excitement in the guide to keep people reading. I won't downvote because you obviously just found Mobafire last night. You need to read Jhoijhoi's guide to make a guide and you need to play a couple hundred more matches and watch some professonal matches to understand why I say your build isnt the best out there.

Your choice of item is very sad.