Where can I find instruction about how to create Modeled Attributes?

We are trying to link a part to a document instead of typing the part number; we would like to a create a modeled attribute to use a “picker.” We just do not know how to do it. I can’t find any instructions about how to create a modeled attribute. All I have found on modeled attributes is this: “Modeled attributes are programmed in Java and cannot be created using the Type and Attribute Management tool”

We would like to know what we need to do it and how we do it.

Thanks!