I have the quest, where to go?
The World Boss (shortened WB in global) spawns in at least 2 different locations on one certain map every month. Here is a list with the past locations:
When a new month hits in, check or ask in Global/Guild for the new location.January: Conall's/Atzel. Boss: The King of Winter (Giant Yeti). Summoner: Ymirish Priest. Yeti in Atzels and Conalls Valley areas (2 spots of each, by the Black Dragon Magus and the top of the Halls of Eternal Frost in Atzels and by the Yeti camp and the Ymirs camp in Conalls).
February: Northern Grasslands/Chosain Province. Boss: Obsidian Basilisk. Summoner: Legendary Monster Hunters. NG was near the Hole, or north of Pin-Pin near the instance bridge, or at the Red Cliff Inn area. Chosain was either near the northern chicken coop rez pad or further east by the pagoda/in the swamp.
March: Dragonspine. Boss: Serpent Man Oldblood. Summoner: Cursed Elder. One spawn point (east) was near the Excavator's Camp/Slithering Coil (1300, 500). The other (west) was west of the Nomad Camp (460, 350).
April: Thunder River. Boss: The Thrice Drowned. Summoner: Followers of the Deep. One spot is at 500, 1300, island in river NW of Croc Caves. 2nd spawn point around 970, 600, south end across from the southern rez pad. 3rd is 1320, 1660, near path to Graveyard.
The World Boss for May (Yahkmar) spawns in Ymir's Pass at Amphitheatre of Karutonia (1200, 795), and Temple of Ymir (940, 1260), in Eiglophian Mountains at Vile Nativity (310, 1210), and Yakhmar's Cave (595, 1165).
The World Boss of June (Spawn of Nergal) spawns in the Coast of Ardashir on the Island (775, 840) and the Logging Camp (660, 470). You can use the respad at the nearby camp to switch instances (660, 320).
The World Boss for July (Leviathus) spawns in Khopshef Province at Crossroads (488, 234), Ruins (288, 375), Black Castle (778, 242), and Caravanserai (792, 554); also Dragons Spine Oasis (560, 970).
The World Boss for August (Black One Executioner) spawns in White Sands at Ancient Temple (718, 607) kiteable to the Beach (respoint at 595-225).
The world boss for September (Remnant of the Lurker) spawns in Eiglophian Mountains at the temple's lobby (170, 1300) and also in FotD at the temple's entrance as well (550, 1245).
The World Boss for October (Banshee Queen of Acheron) spawns in Wildlands of Zelata F2P, L20-40 zone AND Thunder River F2P, L60+ zone (confirmed). Banshee Queen of Acheron. Summoner: Black Ring High-Necromancer. Spawns at Wildlands of Zelata at Sanctum of the Burning Souls (1200, 630) and in Thunder River at Xibaluku (920, 960). Wildlands spawn site is quite close to a rez pad in the ruins.
The World Boss for November (Risen Inferno Lord) Spawns in Kheshatta. Summoner: Flame Oracle. 4 spawn locations (all in Keshatta): East Ruins (1125, 865), Caravan Raiders Camp (1130, 435), Ghanatan Camps (650, 155), and Black Ring Explorers (260,640).
I'm there, what to do now?
The world boss spawns once randomly every 30 to 60 minutes, so for a proper farm you will have to hop from instance to instance. The instances spawn depending on how many players are farming. The lazy way is to check global for a WB line like "lector, zeta, island" which translates in "(spawning npc), (instance), (exact spawn location)". The chances are you might not make it in by the time it's called though. The non lazy way is to keep track of the last spawns for an accurate guess of the next locations, so you can just write down the spawning instances as they come and switch beforehand to the one that could be next (for example: Balder, Zeta, Gamma, Nu, Balder, Theta, Zeta, Nu, Gamma, Theta, Zeta ...). If you have a large group, you might even want to contribute to the world by scouting the next possible locations.
What about the rewards?
You need to be on the Boss's threat list to get a guaranteed drop ("Defender of Hyboria" buff should be enough but with the new hotfix I'm not sure anymore). The chest will automatically appear in your bag, make sure you have place! (note that since patch 4.2.1 you will need to be at least 75m from the boss for getting any loot). If you are in a hurry, I recommend you help take down the boss instead of going afk - since the hp of the WB escalates depending on the people that hit it, the more slackers means the longer it will take to down it (and thus the longer you will have to wait before you can port to a new spawn).
There are two basic drops, one Blue Chest (name changes depending on the boss) and an Epic Chest. The epic chest contains usually [Simple Relics III], and only rarely a pet in the looks of the WB you killed. For more details on the breakdown of the chests there are a couple of websites out there, but basically it's all vanity, potions, companions and t1/t2 gear (and rarely even an epic chest inside the blue one).
For Crom's sake, never kite the boss. If you get aggro die like a man.
Once you get the hang of it it's: switch instance, kill, switch, kill, switch, kill (with a bit of waiting in between). If you have any question regarding WB let me know.
Have fun hunting!