Add the 'Notes and Attachments' related list to existing Custom Objects

To add a "Notes & Attachments" related list to an existing custom object, follow the steps listed below. 
  

1. Click on:
   • Classic: Click Setup | Build | Create | Objects.
   • Lightning: Click  Gear Icon | Setup | Object Manager.
       
2. Click the name of the desired custom object.
3. Scroll down to the page layouts related list, and click the "Edit" link in the Action column next to the appropriate page layout name.
4. Select the Related List option from the palette on the upper portion of the screen. Then drag and drop the Notes and Attachments Related List to where you would like it on the Pagelayout.
     
   • Note: Should you have any questions, please refer to Customizing Page Layouts with the Enhanced Page Layout Editor:
       
5. Click Save.