Bit of a job to explain mate really. If the original undertray has fallen off i would guess that most of the female fixings will still be fixed onto the chassis, May become more obvious when you get it up in the air
The white washer type pieces push directly onto the large black plastic fixings, which then push through the undertray and fix by half a turn into the chassis
I think that the smaller metal black screws incorporated with the bright female fixings may fix the tray on along the line closest to the bumper. Also there is a fixing for the tray inside both wheel arches at the very bottom , these could be for that too.
As i say mate..... bit hard to explain. Good luck