get 6 1/2 ft of non stretchy string and a pencil



and a 6 ft board and a few screws



screw the 6 ft board into the ply wood so it looks like my picture.



make sure there is an inch of space on the ply wood between the board.



now for the string



go to the top of the board and measure one inch down from the top and but a screw in to it.



now tie your string onto the screw



and on the other end tie a pencil make sure the pencil is perfectly on the bottom of the ply wood as seen in my picture.



now keep the string pulled taught ,the pencil strait up and go up and down a couple times until you get a nice smooth line. that looks like mine.



now if you're going to use coping or not draw a little notch in the top to put your coping into if you're using a 2 inch tube make the notch 1 1/2 inches each way.