Please refer to below steps to relate one document to another.

Method 1

To relate documents together highlight one or more documents that you want related to the other document. Then press Ctrl-c on the keyboard to perform a copy.
Right click on the second document you want to relate the first document(s) to and select Document Info -> Related Documents.
The Related Documents window will appear. Press Ctrl-v to add the first document(s) to related documents window which will relate the documents together.
Close the related documents window when done. Please refer to attached images for more information.

Method 2

Right click on the first document you want to and select Document Info -> Related Documents. The Related Documents window will appear

Change the size of the Outlook window so you can see the related documents window behind it.
Drag and drop the one or more other documents to the related documents window to relate the documents together.