Hello, I am ClutchSkit, a EUNE Silver 2 Varus Main! I have 250k mastery points, and I have been playing since the end of season 7. I'm still learning new stuff so I'll update this guide when I find some good stuff.
I decided to make this guide because it's a fun champ.

In my opinion it is best to ban Caitlyn, Ziggs or Lucian. These are champions that can keep you at 50 farm in 20 minutes with the pressure. Caitlyn can pretty much one shot you, even under turret with her traps. Ziggs can poke at you every few seconds, just getting your health down continously. Lucian can one shot you easily, especially with the proper support.

If you get W first you get a lot of dmg early game especially level 1. This works best with supports that have cc at level 1, you can get the enemy to half hp at level 1, if your support can hit their cc. Q is a good pick for the first upgrade because of the poke, and also the synergy with your W. The more you charge the Q, the more damage the W stacks do. If you activate W, his Q works like an execute, dealing missing health damage.
One thing to always remember, enemies going through the area left behind by your E get slowed, and also have reduced healing. Good against champions with a lot of healing obviously, but it also means that using it late, when the support heals the ADC after a fight, can help you keep the ADC low. The slow is great for escaping champions with no movement abilities.

Early game you wait for support to cc, 3 AA to get PTA, and then, depending on the enemy's health, either W-Q, or just Q. If you are level 3, you can use E and aim it so it is mostly behind the enemy, so you can continue hitting them, thanks to the slow.
If your enemies are grouped up and they are pushing, get your teammates, signal the fact that your ult is up, and then Ult, and if you have Runaan, you can get Blight on 3 enemies, since they are probably going to be rooted by your ult and you can deal a lot of damage to all of them using your E, and following up with a W, and a Q, for enemies that are low hp.

If you go Full AP, get Magic Pen boots, and void staff, you could steal Baron/Drake with your W-Q.

Thank you for reading, and I hope you enjoyed this guide! I know it doesn't look as good as other guides but I just wanted to help you use a build that helped me win a lot of games!