• Talk to Fyi Chalmwoh in Goldsmithing Guild in Mhaura, see cutscene, talk to her again.
• Talk to Ramona in Weavers Guild in Selbina, see Cutscene, get Key item 'Ye Olde Mannequin Catalog'.
• Cheupirudaux (F-3) at the Carpenters Guild in Northern San d'Oria sends you to get 6 mannequin parts from Beastmen.

 You will have to do the Garrison encounter for a chance to get the mannequin parts.
 What is the Mannequin encounter? I will explain it all here.
 
Garrison Encounters: 

You will need 1 of 3 different items to trigger the Garrison encounter. The item you need drops off of the same mobs that you fight for Kazahm Keys.

• If you have not done the Kazham key quest yet, the mob names are given in the Garrison Info on the Allakhazam site.

These mobs can be found in other areas and should drop the garrison item, eg. Bastokians can hunt anywhere they can find Brass, Copper, and Old Quadavs.
- You only need one item for each time your party/alliance wishes to participate in garrison.
 
 Make sure that the person that receives the item is of that realm or it will not work.
 
 Make sure that the person who trades garrison item to guard has started the mannequin quest
 and has talked with Cheupirudaux in N. Sandoria.
 
For Windurst you need a Seven Knot Quipus from Giddes and trade to the W. Sarutabaruta guard.
For Bastok you need a Darksteel Engraving from Palborough mines and trade to the North Gusta guard.
For Sandoria you need a Red Cryptex from Yughott Grotto and trade to the W. Ronfaura guard.
 
Once you do this, about 10 NPC's called 'Patrician' will spawn. They're here to help you fight. They're mostly inconveniences that will get killed unless you keep them healed up, and you'll want to keep them healed up. If all of them die, fight's over, and no one gets a reward. Now, I should note that the Mannequin pieces are not part of the reward for completing the battle, they are just drops off the mobs you will fight during the battle.

Oh, and you are also applied with a Level Cap of 20. So bring appropriate gear.

About 10 seconds after the friendly NPC's appear, the first wave of 5 beastman will spawn. The mobs you fight are listed on Allakhazam, so I won't repeat them here. However, after you kill them all, there's about 10-15 seconds of downtime before the next set spawns. You'll want your mages to use juices here, and make sure your tanks/dd's are healed up.

Another wave will spawn, and they'll go to town. Kill em all, wash, rinse, repeat. Make sure you keep patricians alive! The more of them that are standing at the end of the fight, the more equipment your group will be given, as well as the gil reward will increase.
 
After 4 waves, a 'boss' monster will spawn. He'll be a bit tougher than the previous mobs, but since you'll have 18 people beating on him, beware. They are 'classed' mobs who do not hesistate to use their two hours. Once you take this 'boss' mob down, you win! Money, items, and festive waffles are passed out. Then you can give them another item and do it again.

You need a minimum of ten players and a maximum of 12. 6 NPC fighters will assist you, too. The more NPCs that you keep alive, the better the drops. EVERYONE gets between 800-2000 gil and a (1) Dragon Chronicle always drops. It also gives you a little bit of Conquest Points.
 
The first wave is easy... probably meant to give you a false sense of security. Kill the Chaussers (Rangers) first then move onto the Fighters.
 

The second wave has a bit more challenge to it. Kill the Chaussers first again and watch for the Cursemakers to pop. When they do, KILL THEM IMMEDIATELY. They Cure and use Poisonga.
 
The third wave is similar to second wave only 2 Sergeants pop as well. Just when you think you're about to win, an Orcish Fighterchief (Monk) will pop and most likely use 100 Fists on the closest enmitied character... killing any mage or poorly healed melee. At this point, you focus all your attacks on the Chief while you have two Vokers taking on the two Cursemakers if they are still there. You CAN save all your TP for Chief, but just focusing your attack should do it if your party is strong enough.
 
Your drops will be dependant on the NPCs left alive. I got 1000 gil, Dragon Chronicle, and Garrison Tunic the first time.

More Notes: 

"Ok this is how it works, mobs spawn in waves, wave 1, 4 mobs, wave 2, 6 mobs, wave 3, 8 mobs and wave 4, 10 mobs. you can have a full alliance for it, and npcs match you, you have 6 healers out of 18 people then npcs bring 6 healers. you want to make sure that npc stay alive. and there is a grace period on doing this. after you beat it once, you have to wait about 10-20 min real time, to trade agian. you can only trade item to your town ex: San can only trdae Red crypt's in West Ronfaure. "

"Here's what I suggest from watching them. You need 2 WHM to cure the NPCs ONLY! 3 Melee: PLD or RNGs are BEST! both with sub WAR to voke off candidates. The last member should be a RDM to heal the melee if they need it.  Use potions! There is a potion to give you 100 life instantly, use it!  Find someone who can make it or something, it's cheaper that way. It's good to have at least 2 of these potions on you. And mana potions for the mages of course. Food is useful too, you can find out what is useful"

"You must be as fast as possible in killing these things, for you have absolutely no chance to rest in between."

"The ones curing the candidates are most important, they have to keep on their toes watching these NPCs, it's very very important."

"Many people think they win if the Yags disapear. No. If they disapear, before you kill them all, that's cause all the candidates died. "
 
"Trade the Darksteel Engraving to the Iron Musketeer at the North Gustaberg outpost, and he will tell you that it details and Quadav attack in North Gustaberg.
-  After the speech, your alliance will recieve a level cap of 20 (you only need one Darksteel Engraving per Alliance, similar to BCNM battles.
-  You will be attacked by 3 (three) waves of Quadavs, and some NPCs named Recruit will arrive to help you fight the Quadavs.
-  The recruits fight the Quadavs as well, and can be healed by PCs. The more recruits that
 survive, the better the loot at the end is.
-  Between each wave, you will get about 10-20 seconds respite before the next wave
 spawns almost right on top of you.
- The number of Quadavs in the waves is dependant on the number of people in your
 alliance. Therefore, taking a full alliance won't make the Garrison any easier. "

More information about Parts: 

Recent info on the forums state that the new BCNMs also drop mannequin pieces.
Here's a list of potential drops:

Mannequin Head: Ghelsba outpost-Cloudy orb bcnm & Balga's Dais-Cloudy orb bcnm

Mannequin Body: Waughroon Shrine-Sky orb bcnm & Qu'Bia Arena-Sky orb bcnm

Mannequin Hands: Balga's Dais-Cloudy orb bcnm & Waughroon Shrine-Sky orb bcnm

Mannequin Legs: Garrison, or a possible quest reward of "Uninvited Guests"

Mannequin Feet: Garrison