The info_overlay entity is automatically placed when you use the Hammer Overlay Tool. The sides keyvalue is automatically set when placed in this manner. If you need the overlay to span multiple faces, open the overlay's properties in SmartEdit mode, choose Brush Faces and then Ctrl-Click in the camera window to add more faces.
The overlay is a more powerful version of a standard decal: it can be resized, rotated, scaled, and distorted. Overlays can also be placed on displacements, where decals can't.
If the overlay is not yet placed, select the overlay tool and your material, and click where you wish the center of the overlay to be. You should now see 4 white boxes at the edges of the overlay, and you may drag them about to distort it.
If you wish to distort an existing overlay, first select the overlay you wish to edit, and then select the overlay tool. Then you should be able to distort the overlay as described above. If the overlay does not respond, try deleting it and then following the above procedure.