Step 1: find in Armor.txt the entry of the item you want to add  
Step 2: Create a .lsx (you can copy the lsx associated to the item)  
Step 2.b: Generate a new uuid V4 and change the Mapkey line inside the new .lsx  
Step 2.c: insert those lines inside the GameObject under the last attribute, if the gameobject already has a children node then insert only the content of the following children node without the node itself:

<children>
<node id="OnUsePeaceActions">
<children>
<node id="Action">
<attribute id="ActionType" type="int32" value="23" />
<children>
<node id="Attributes">
<attribute id="Animation" type="FixedString" value="" />
<attribute id="Conditions" type="LSString" value="" />
<attribute id="CombineSlots" type="int8" value="1" />
<attribute id="IsBase" type="bool" value="True" />
</node>
</children>
</node>
</children>
</node>
</children>

Step 2.d: Change the value in  Stats line and the Name line by Something you want but it needs to be the same thing for both lines
Step 3: Create an Armor.txt and insert inside :

new entry "NAME/STATS YOU CHOOSE AT STEP 2.D"
type "Armor"
data "RootTemplate" "UUID YOU GENERATED AT STEP 2.B"
data "Slot" "None"
data "ArmorClass" "0"
data "Boosts" ""
data "PassivesOnEquip" ""
data "Rarity" "Common"
data "Weight" "1"

Now you have the appearance Item Created you need to create the Combo to get it
For the Combo you need to create a ItemCombos.txt and insert :

new ItemCombination "NAME YOU CHOOSE "
data "Type 1" "Object"
data "Object 1" "WAY_Appearence_Remover"
data "Combine 1" "Base"
data "Transform 1" "None"
data "Type 2" "Object"
data "Object 2" "NAME OF THE ORIGINAL ITEM STATS/NAME"
data "Transform 2" "None"

new ItemCombinationResult "SAME NAME YOU CHOOSE but with _1 at the end"
data "ResultAmount 1" "1"
data "Result 1" "NAME/STATS YOU CHOOSE AT STEP 2.D"
data "PreviewStatsID" "NAME/STATS YOU HAVE CHOOSEN AT STEP 2.D"
data "PreviewIcon" "VALUE FROM THE ICON LINE IN THE ORIGINAL ITEM .LSX"


With Your new Appearance item and your new way to obtain it you can now try to combine it and be disapointed to see it doesn't work with any item, it's because you have to create an Armor Stats and a itemcombo for every possible combination
So in the Armor.txt Previously created you need to insert those lines:

new entry "NEW NAME YOU CHOOSE"
type "Armor"
using "NAME/STATS OF THE ITEM YOU TO TRANSFORM"
data "RootTemplate" "THE ORIGINAL UUID OF THE ITEM YOU WANT TO TRANSFORM TO" (ciri's boots uuid for exemple if you want the item to have ciri's boots appearance)

then you have to create the Combo to obtain this new item
So in the ItemCombos.txt Previously created you need to insert those lines:

new ItemCombination "NEW NAME YOU CHOOSE"
data "Type 1" "Object"
data "Object 1" "NAME/STATS YOU CHOOSE AT STEP 2.D"
data "Combine 1" "Base"
data "Transform 1" "None"
data "Type 2" "Object"
data "Object 2" "THE STATS/NAME USED BY THE ORIGINAL ITEM YOU WANT TO TRANSFORM"
data "Transform 2" "Consume"

new ItemCombinationResult "SAME NAME YOU CHOOSE but with _1 at the end"
data "ResultAmount 1" "1"
data "Result 1" "THE NEW ITEM STATS/NAME YOU ADDED TO ARMOR.TXT"
data "PreviewStatsID" "THE NEW ITEM STATS/NAME YOU ADDED TO ARMOR.TXT"
data "PreviewIcon" "ICON OF THE ORIGINAL ITEM YOU WANT TO TRANSFORM TO"(ciri's boots icon for example)

So now you Have created your appearance, created a way to get it, created a new item using the stats of the item you want to transform and the appearance of the ciri's boots and added a combo so you can combine ciri's boot's appearance to get the new item
now you have to add a new combo to be able to remove ciri's appearance :
So in the ItemCombos.txt Previously created you need to insert those lines:

new ItemCombination "NEW NAME YOU CHOOSE"
data "Type 1" "Object"
data "Object 1" "WAY_Appearence_Remover"
data "Combine 1" "Base"
data "Transform 1" "None"
data "Type 2" "Object"
data "Object 2" "STATS/NAME OF THE ITEM YOU CREATED JUST BEFORE"(not the appearance but the second item you added to armor.txt)
data "Transform 2" "Consume"

new ItemCombinationResult "SAME NAME YOU CHOOSE but with _1 at the end"
data "ResultAmount 1" "1"
data "Result 1" "ORIGINAL NAME/STATS OF THE ITEM YOU WANTED TO TRANSFORM"
data "PreviewStatsID" "ORIGINAL NAME/STATS OF THE ITEM YOU WANTED TO TRANSFORM"
data "PreviewIcon" "ICON OF THE ITEM YOU WANTED TO TRANSFORM"