How do I transfer My Documents to the D Drive?

You can create D:\Documents or D:\Data\Docs and the folder(s) to Windows File Explorer's new "Quick Access" area.

But to truly remap the "My Documents" folder, see this:

http://www.thewindowsclub.com/change-default-save-location-windows-10 

Edited

  I wonder if in Computer Management if you have created a Data D:  "docs" and assign it 'mounted' to C: "docs" if there

then is the option (whenever u save something) to 'save it'  in D: "docs".    That might be cool.  And the prompted option for D: "docs" would help remember not to have autosave to C: "docs".