To use custom blocks in Drupal 8, follow these steps:
1. Create a custom block by going to Structure > Block layout > Custom block library > Add custom block. Give it a title and fill in the content.
1. Go to Structure > Block layout.
1. Choose a region where you want to place the custom block.
1. Click on the Place block button.
1. In the search bar, type your custom block’s title, then click on it to select it.
1. Click on the Add block button.
1. You can then configure the placement of the custom block, such as specifying which pages it will appear on.
1. Save the changes.
Your custom block should now be visible on your Drupal 8 website.
