Use the reflective side to reflect light inward and tape the dull side edges with duct tape to give the foil strength.
You can see from the picture I have foil across the top of two light fixtures. It covers the gap and reflects the light back down.
I have flaps in the front an back that I can move. The front flap is of course closed in the picture. I also use side flaps. It doesn't have to be perfect but by doing this you really reflect a lot of lot back toward your plants.
|NO Foil Used to Reflect Light|
|Foil Flaps and Covers|