So the picture is from someone else's deck, maybe off the 'net, and you want to duplicate it? It looks like that one was intended to be all 45's and 90's, but has some alignment issues. If the posts are all spaced evenly, you could take the inside dimension of them, then measure down from the top rail to determine where the top of the bottom rail would be positioned. That would give you squares to work with, and you would use all 45's and 90's. Otherwise, if the spacing between posts will be different than the spacing between the rails, making a rectangle, hold a board diagonally, corner to corner, mark and cut it, make sure it fits, then you use a protractor to get your angles. The angle where they hit the rails and the angles that hit the posts will add up to 90, so if one is say 52 degrees, the other will be 38 degrees, but the butt ends will remain 90.