Talk to any of the vampires in Castle Volkihar, such as Orthjolf,  to find out who has the next mission for you. They point you to Feran Sadri who has something for you. When you ask Sadri how you can help, he tells you that the Dawnguard have increased their operations and have been aggressively recruiting. We need to put a damper on their efforts and give those trouble makers something to worry about other than harassing our interests.

You are to make a public show of murdering someone. Do it in the streets for all to see. The higher the profile, the better. Make it seem you are a member of the Dawnguard, thus sowing confusion among the herd. Wear this armor and plant this note on the victim.

Ask Feran what he means by "high profile target". He says it means an important member of society, someone attached to the Jarl's court, a merchant, a bard, a priest, someone with stature. That will make them all more upset to have one of their citizens slain, and under such flimsy pretense.

This starts the quest:

Deceiving the Herd
Feran Sadri wishes to sow confusion and anger amongst the people of Windhelm. I am to kill a citizen out in the streets while impersonating a member of the Dawnguard. Then I am to place an incriminating letter on the corpse. The higher the profile of my victim the better.

(Optional) Choose a high profile victim
(Optional) Make it a public kill while wearing Dawnguard armor
Kill someone out in the streets of Windhelm

You should also see the following added to your inventory:
Dawnguard Boots added
Dawnguard Gauntlets Added
Dawnguard Full Helmet Added
Dawnguard Heavy Armor Added
Incriminating Letter Added

Note: The city is chosen at random. It will be one of the Hold Capitals. So for example instead of Windhelm, you could get Markarth, Dawnstar, Falkreath, Morthal, Riften, Solitude, Whiterun, or Winterhold

Put on the Dawnguard Armor and go to the city you were assigned. Locate a high profile target. It is best to use people from the list below, because you might think that a Thane or a Jarl's family member is high profile, but often they are not and the objective will fail. The list below has CONFIRMED high profile targets. If you discover more high profile targets, please leave a comment with details so that everyone can benefit!

Here is a list of some potential targets that are high profile:

Deceiving the Herd Dawnstar High Profile Victims
Frida (Mortar and Pestle)
Leigelf (Quicksilver Mine Owner)
Madena (Court Wizard)
Silus Vesuvius (Priest)
Thoring (Innkeeper, Windpeak Inn)

Deceiving the Herd Falkreath High Profile Victims
Lod (Blacksmith)
Solaf (Falkreath, Gray Pine Goods)
Valga Vinicia (Innkeeper, Dead Man's Drink)
Zaria (Alchemist, Grave Concoctions)

Deceiving the Herd Markarth High Profile Victims
Bothela (The Hag's Cure)
Hogni Red-Arm (Cannibal who sells meat)
Ghorza Gra-Bagol (Blacksmith)
Kerah (Jewelry Stall)
Lisbet (Store Owner)

Deceiving the Herd Morthal High Profile Victims
Falion (Mage)
Jonna (Innkeeper, Moorside Inn)
Lami (Thaumaturgist's Hut)

Deceiving the Herd Riften High Profile Victims
Brand-Shei (Shopkeeper)
Grelka (Merchant)
Keerava (The Bee and the Barb)
Madesi (Jeweler)
Marise Aravel (Merchant)

Deceiving the Herd Solitude High Profile Victims
Angeline Morard (Angeline's Aromatics)
Beirand (Nord Blacksmith)
Corpullus Vinius (The Winking Skeever Innkeeper)
Evette San (Stallkeeper)
Fihada (Shopkeeper, Fletcher)

Deceiving the Herd Whiterun High Profile Victims
Anoriath (The Drunken Huntsman)
Adrianne Avenicci (Warmaiden's)
Belethor (Storekeeper, Belethor's General Goods)
Carlotta Valentia (Food Vendor)
Danica Pure-Spring (Priestess)

Deceiving the Herd Windhelm High Profile Victims

Aval Atheron (Street Merchant)
Calixto Corrium (Calixto's House of Curiosities)
Hillevi Cruel-Sea (Farmer)
Jora (Priestess of Talos)

Deceiving the Herd Winterhold High Profile Victims
Birna (Birna's Oddments)
Dagur (Frozen Hearth Innkeeper)

Unlike some previous missions, this is NOT a stealthy (discreet) assassination. You WANT to kill the mark in plain sight, with as many people as you can witnessing this crime. And yes you will get a bounty.

After killing your target, immediately approach the body and search it. From your inventory, you search under BOOKS for the Incriminating Letter and press the button you use to STORE things, to plant the letter onto your victim's body. At this point guards will be attacking you, and you may want to escape the city as quickly as you can and without killing further people, so as to not accrue a higher bounty (Another option if you are powerful enough is to lure all witnesses to some location, transform into a Vampire Lord, and kill all of them thus removing your bounty)

Once you are at a safe distance from the city, fast travel to Castle Volkihar and talk to Feran Sadri. Tell him you have slain someone on the streets of Windhelm. Feran hopes this will direct the people's anger towards the Dawnguard and away from the Vampires. If you chose a high profile target, he congratulates you further, and gives yet additional praise for carrying out his instructions to kill the victim in view of others while wearing the Dawnguard uniform. He gives you some potions of blood as a reward.

This completes the quest, Deceiving the Herd.