How do I configure the REST API?
I search a lot too after 8.2x and this is how i got it working :

Example for a custom user ressource :


  myModule/config/install
  
  rest.resource.user.rest_ressource.yml


langcode: en
status: true
dependencies:
  module:
    - basic_auth
id: user.rest_ressource
plugin_id: 'user_rest_ressource'
granularity: resource
configuration:
  methods:
    - GET
  formats:
    - json
  authentication:
    - basic_auth



  In myModule/src/Plugin/rest/resource
  
  UserRestRessource.php


&lt;?php

namespace Drupal\myModule\Plugin\rest\resource;

use Drupal\Core\Session\AccountProxyInterface;
use Drupal\rest\Plugin\ResourceBase;
use Drupal\rest\ResourceResponse;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Psr\Log\LoggerInterface;


/**
 * Provides a resource to get view modes by entity and bundle.
 *
 * @RestResource(
 *   id = "user_rest_ressource",
 *   label = @Translation("User Rest"),
 *   uri_paths = {
 *     "canonical" = "/api/user/getInfo"
 *   }
 * )
 */
class UserRestRessource extends ResourceBase {

  /**
   * A current user instance.
   *
   * @var \Drupal\Core\Session\AccountProxyInterface
   */
  protected $currentUser;

  /**
   * Constructs a Drupal\rest\Plugin\ResourceBase object.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param array $serializer_formats
   *   The available serialization formats.
   * @param \Psr\Log\LoggerInterface $logger
   *   A logger instance.
   * @param \Drupal\Core\Session\AccountProxyInterface $current_user
   *   A current user instance.
   */
  public function __construct(
    array $configuration,
    $plugin_id,
    $plugin_definition,
    array $serializer_formats,
    LoggerInterface $logger,
    AccountProxyInterface $current_user) {
    parent::__construct($configuration, $plugin_id, $plugin_definition, $serializer_formats, $logger);

    $this-&gt;currentUser = $current_user;

  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container-&gt;getParameter('serializer.formats'),
      $container-&gt;get('logger.factory')-&gt;get('restimadiff'),
      $container-&gt;get('current_user')
    );
  }

  /**
   * Responds to GET requests.
   *
   * 
   *
   * @throws \Symfony\Component\HttpKernel\Exception\HttpException
   *   Throws exception expected.
   */
  public function get() {

      $responseResource=new ResourceResponse(
          array()

      );  
    return $responseResource;
  }

}