Follow the steps in the generic item guide first to define the other required properties of your item.
The CoreLib.Equipment submodule is required for this guide section.
To make armor you need to:
Set the ObjectType to armor type
Add DurabilityAuthoring, ModEquipmentSkinAuthoring, GivesConditionsWhenEquipedAuthoring and LevelAuthoring component authorings and configure them correctly
Make a armor spite sheet. Examples of such sheets can be found in the Unity Editor. Now assign your texture to ModEquipmentSkinAuthoring (CoreLib feature). The component will automatically set everything up.