For making a custom block, the steps which are described below are required:

Step 1: In your Drupal 8 folder that you have installed you have a folder named module in that module you have to make a sub-folder named custom.

modules/custom

Step 2: Next step is to create a sub folder in that custom folder that you have recently created.The folder structure goes like this.

Step 3: The next step is to create info.yml to make the custom block. In that info.yml, you have to mention the name of the block, description of the block, core, package, dependencies, type.

name: TestArticle Block
description: A simple module to demonstrate Block.
core: 8.x
package: Custom
dependencies:
- node
- block
type: module

This file should be saved according to the name that you have created inside the custom folder.

Step 4: After that, you have to create a file with .php extension the name of the file should be same to the name that you have created inside the custom folder.

<?php
/**
 * @file
 * Contains \Drupal\article\Plugin\Block\XaiBlock.
 */
namespace stonesdrupal\testarticle\Plugin\Block;
use stonesdrupal\Core\Block\BlockBase;

class TestArticleBlock extends BlockBase {
  /**
   * {@inheritdoc}
   */
  public function build() {
    return array(
      '#type' => 'markup',
      '#markup' => 'This block list the article.',
    );
  }
}

After that you have to give the permission to the all the folder that you have created. Then your custom block is created.

Step 5: You have to now place the block to the particular region that you want.