I accidentally delete my downloads stack. How do I restore it back?

Find your downloads folder in Finder ~/Downloads

Drag the downloads folder onto the dock in the location you desire.

If the folder does not appear as a stack, right click on the icon on the dock and select "Stack"

~/ represents your home directory. It is accessible from the "Go" menu of Finder.

Use the search function (command + spacebar), type in "downloads" and drag the folder back into finder.