All you need to do is cut a panel out of the board the size of the vented section of the vent, then screw the vent over it. I marked the rectangle with a pencil, used a drill to make holes in the corners, then a jigsaw to cut the lines. Nice and easy :)
:Question - you know you're ventilattion panels you have on the back of your viv? How did you put them in? I've just finished building a 4'x2'x2' viv for my bearded dragon (and will be building another for my 2 Uromastyx) and I think I need those but dunno how to fit em!