Here is a very basic step by step guide - assuming you have at least opened the data editor once at some point
(Note: Things might actually be called different in the editor, i am using the german editor so i can only assume what it is called in the english version of the editor )
1. In the Data Editor: Go to Data-> Create doodad
Set a name, doodad type, base size (this is important for selecting and moving your doodad in the editor)
2. Open the "Models" tab, find the doodad you have just created.
Find the field that says model, double click it, and browse the the folders (Buildings, Units, etc.). Pick any model file (*.m3)
3. Place it on your map
Here's an example with a 500% size Point Defense Drone doodad floating behind/under the space platform.