For me it was much easier to sight down the centerline than it was the sides (wl's). So i notched the WL marks and ran a thin string through the notches on either side. Made it much easier to see any dips or rises. I also used a air nailer to tack (leaving the nail out by about .25") the forms to the supports before going back and attaching them with a screw gun. This only helped to avoid having the forms move while driving the screws, and isnt so important.

Keep in mind that tacking with an air nailer can be dangerous. I fit a scrap of strip with a .25" hole in it over the end of my gun so that I was shooting through the hole. That meant there was no gap between the gun, then strip, then workpiece. That way if a nail went wild, it wouldnt fly across the room.