The Migrate Plus module also comes with some examples of how to build and manage migrations. Once you download the Migrate Plus module using drush dl migrate_plus, the examples are available in modules/contrib/migrate_plus/migrate_example.

The first step is creating a migration group, which is found in the config/install directory for the module for the Configuration Entity to be created when the module is installed. The migration group has basic attributes for a machine name, label, description, and source description:

id: beer 
label: Beer Imports 
description: A few simple beer-related imports, to demonstrate how to implement migrations. 
source_type: Custom tables 
shared_configuration: 
    key: default 

Once you have the migration group defined, you would add individual migrations to it:

# Migration configuration for beer content. 
id: beer_node 
label: Beers of the world 
migration_group: beer 
source: 
  plugin: beer_node 
destination: 
  plugin: entity:node 
process: 
  type: 
    plugin: default_value 
    default_value: migrate_example_beer 
  title: name 
  nid: bid 
  uid: 
    plugin: migration 
    migration: beer_user 
    source: aid 
  sticky: 
    plugin: default_value 
    default_value: 0 
  field_migrate_example_country: countries 
  field_migrate_example_beer_style: 
    plugin: migration 
    migration: beer_term 
    source: terms 
  'body/value': body 
  'body/summary': excerpt 
migration_dependencies: 
  required: 
    - beer_term 
    - beer_user 

In the configuration definition, it defines the migration group that it is part of, the source migration plugin, the target entity that will be created or updated, and then how values from the source are mapped to it. For example, the field title in the node will be populated from the name attribute from the migration plugin. In addition to basic mappings, you can also add different plugins to process the result. These include:

• Callback: It triggers a callable with the value
• Concat: It concatenates an array of values
• DefaultValue: It sets a default value for missing values
• Explode: It converts a delimited string into an array of values
• Extract: It returns a single value from an array
• Flatten: It flattens a nested array to a single depth
• Get: It applies a regular expression to the value
• MachineName: It constructs a machine name from a value
• SkipOnEmpty: It skips the current field or row if the value is empty
• SkipRowIfNotSet: It skips the current row if the value is empty
• StaticMap: It maps the value to another value