1: A NPC.

2: An armor/cloth in the chest slot of that NPC.

3: The On_Spawn script : p_cf2_npc_spawn in the NPC On_Spawn slot or make your own and use the following code in your own script.

#include "inc_appr"

void main()
{
    string sBodyAppearance = GetLocalString(OBJECT_SELF, "P_BODY_APPEARANCE");
    string sArmorAppearance = GetLocalString(OBJECT_SELF, "P_ARMOR_APPEARANCE");
    string sHeadAppearance = GetLocalString(OBJECT_SELF,"P_HEAD_APPEARANCE");
    object oArmor = GetItemInSlot(INVENTORY_SLOT_CHEST, OBJECT_SELF);
    int iOrginalAC = GetItemAC(oArmor);
    
    if (sBodyAppearance != "")
        {
        SetEntireCreatureAppearance(OBJECT_SELF, sBodyAppearance);
        }
    if(sArmorAppearance!=""&&GetIsObjectValid(oArmor))
        {
        RestoreItemAppearance(oArmor, sArmorAppearance);    
        object oNewArmor = UpdateItemAppr(oArmor);
        if(GetItemAC(oNewArmor)!=iOrginalAC)
            {
            SetItemAC(oNewArmor, iOrginalAC);
            }
        } 
    if(sHeadAppearance!="")
        {
        SetHeadItemOverrideString(OBJECT_SELF,sHeadAppearance);
        }   
}

4: The Variables on the NPC.

Notes : The Body appearance is optional as that would only show if the NPC gets undressed.