You have to dress up a mannequin.
To put pants on a mannequin, you have to remove the base, put the pants on, replace the base.
To put a jacket/shirt on a mannequin, you have to straighten out the arms, put the clothing on, then re-pose the arms.
An armor stand that's basically a T , you just draqe things over.
Realistically speaking, but this is a game. All we have to do is walk up to the mannequin, activate it, and select which armor/clothing to put on the mannequin to place it on.
That's how it was done in The Elder Scrolls IV: Oblivion's Knight of The Nine expansion. After the player got all of the pieces of the Crusader armor, they could walk up to the armor stand, activate it, and take or place certain parts of the armor (and it's weapons) on the stand.