What provide cover? Just give them something to hide behind, a few sandbag tiles or an object.
Many objects are flagged 'V' for cover but they should duck behind short walls and sandbags as well.
Edit Sorry posted that when I just woke up, most actors should be set to use cover by default.
In the level editor, once you've placed your actor press F4 to edit the entities, and then click on an actor. On the left you'll see a window with the entities properties, if you click anywhere in the grey background and keeping your mouse button pressed you'll be able to scroll up and down. The use cover option is in the Tactics section below Nature.