How to position parts relative to each other in Kicad?

The easiest way is to use the "Position Relative To" command. On a Mac, it is ⌘-R, under Linux, it is Ctrl-R. Or, you can access it using the right-click on a part or group.

Once you have selected this, you can choose an object for the reference

You will need to be running version 5 or higher for this function.