I just wanted to write this up really quick because some people create units in an odd way on their tutorials. The steps should be much simpler.
Before getting started: Make sure your data editor views are adjusted accordingly
Create the unit
To create a unit: Open your object editor, go to Units. Add a new object (right click, or Data menu).
Choose whichever name you would like to name your unit, and assign it an ID. It doesn't matter what it's called, don't create the unit just yet. Next, go to Field values, and copy from whichever unit you would like to base it off of. I chose the marine, but any unit should work.
Afterward, you will notice your unit appears on the editor, but it has no model. This is an easy fix and people tend to make it complicated. There is NO reason to create a new model for your one unit, they're already in the game so do not create any new models.
Creating the Actor
Open your Actors data-type now, and add a new object. Preferably name it the same as your custom unit, and set the field values to copy the field values of the unit you want to copy. Next, you're going to have to actually start editting the data of the actor now:
You need to link the actor to your custom unit, so change the field data for the unit name to the created unit you made earlier. Now, the model field values are going to change to that instead of staying the same. This is why the model tends to break and people get confused.
Change the model fields back to the model they were (model fields are named after the models data fields, but they usually have the same names as the object they are linked to, for example if you want your unit to look like a marine, you change the model field to a marine.)
and you're done. If you want me to help you with anything else in the data editor, feel free to ask. Give feedback if you want to also.