Lay it by eye. If it's not over say 50 feet. Find your high point and get down and look down the rail and have a buddy wedge it up where it needs it until it looks consistent. You may have to go to the other end and look back if it's hard to see the whole distance. You can also take a 4'level and place.it on each joint and raise the sections until they are flush.with the bottom of the level. Then go side to side. Over 50 or 60 feet you can use a laser or a string of you can't see it so good. If its a big slope you can bring up the low end so you aren't killing yourself on the move.