Go to WordPress Dashboard > Crocoblock > Theme Builder. 

Click the “Create new page template” button to create a new item.

In the displayed conditions pop-up, press the “Add condition” button.

Then, in the first drop-down list, pick the “Include” option; in the second list, find the “Single” option, and set the third selector to the “404 page” option.

Hit the “Create” button to save the changes.

The created template will be displayed only when there is a 404 error on the page.

Initially, the template is empty; you can see the “Add header,” “Add body,” and “Add footer” buttons that allow creating the needed template sections separately. 

After pressing one of these buttons, two options are displayed. Choose “Create template” to build an item from scratch or “Add from library” to select the already made template.

Another option is to leave some of the elements empty; these are not obligatory to be added. For example, only the body can be displayed, or the header with the body.

We will use the already-built header and footer by clicking the “Add from library” button and selecting the needed items.

Then, we will build a brand new body by hitting the “Create new template” button in the body section.

In the opened pop-up, pick the desired editor in the Template content type field and fill in the Template name field if desired. 

However, the last field is optional; you can freely leave it empty to generate the name automatically.

Push the “Create” button to begin setting up a template.

Click the “Go to editor” button to open the page builder.

Work on the page, add the desired widgets and style the template. Press the “Update” button once you are satisfied with the result.

Check the front end. For instance, mistype the letter in the URL to see the newly built 404 page.