How can we configure custom UOM and UOM Class for an item?

Summary:

Hello,

My requirement is that, we want to define our own UOM class and UOM codes. We want to use these UOM codes as our primary UOM code to the items. That means, we will receive and ship the items in our custom UOM code only. After defining our customer UOM codes and class when i try to assign the custom uom code to already existing item as primary UOM it throws some configuration error. Can you please provide us some document on how to define and configure the UOM which is more helpful. Not able to get the proper documentation