Unzip the zip file. It contains the Theme Builder Global 404 Page Pack 1 and a backup of the CSS used. You will only need that backup file if any of the code gets corrupted on import.
Navigate to Divi > Divi Library. Click the Import & Export button at the top of the page.
Click Import, then Choose file. Locate the Week 148 - Theme Builder Global 404 Page Pack 1.json file on your computer
Be sure to tick Apply Layout's Defaults to this Website. Then click Import Divi Builder Layouts.
When the import process is complete, you should see five new layouts in your library.
Step 3 - Adding the 404 Layout
Navigate to Divi > Theme Builder and click on the Add New Template box. In the Template Settings window that pops up, select 404 Page and then click Create Template.
If you have a global header/footer set already, these will be added to your new template. Next, inside the template we just created, click Add Custom Body, and then Add from Library.
Click Your Saved Layouts, then choose the Global 404 Layout.
You will then be taken back to the Theme Builder and see the layout has been assigned as the Custom Body. Click Save Changes.