Set one box down (double, single, doesn't matter). The flip the bridge on its side, and press it against the box and ground, drop it, your done. If you want it the be vertically tall, then spawn a double box vertically tall, and just place the bridge vertically. If it needs to be floating, float the bridge to the height you want (Make sure your box brace is tall enough) press it against your brace, and align it as parallel to the ground as you can (unless you want it at a slant, save, and quit.