example.json

{
  "data": [
    {
        "id": "1",
        "title": "foo"
    },
    {
        "id": "2",
        "title": "bar"
    },
    {
        "id": "3",
        "title": "baz"
    }
  ]
}  

migration.yml

source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  urls: /sites/default/files/example.json
  item_selector: 'data'
  fields:
    -
      name: machine_name
      label: 'Unique position identifier'
      selector: id
    -
      name: friendly_name
      label: 'Friendly title'
      selector: title
  ids:                                                                                                                
    machine_name:                                                                                                     
      type: string