def METHOD_NAME(self, schema):
    # Add our custom country_code metadata field to the schema.
    schema.update({
            'country_code': [tk.get_validator('ignore_missing'),
                tk.get_converter('convert_to_tags')('country_codes')]
            })
    # Add our custom_test metadata field to the schema, this one will use
    # convert_to_extras instead of convert_to_tags.
    schema.update({
            'custom_text': [tk.get_validator('ignore_missing'),
                tk.get_converter('convert_to_extras')]
            })
    # Add our custom_resource_text metadata field to the schema
    schema['resources'].update({
            'custom_resource_text' : [ tk.get_validator('ignore_missing') ]
            })
    return schema