It's important to get the spacing of your battens correct. Measure the width of the carport using the front rafter and work out where you will place the battens. They need to be evenly spaced and the correct distance apart for the kind of roof you're attaching. With a pencil, mark on the beam where each of the battens will be secured.
Run a string line from the high side of the carport to the lower side. Do this by hammering in a nail on the second rafter on the high side of the carport and another on the opposite side. Attach a string line at the bottom of the two nails and make the line tight. This string line will tell you the height the battens need to be cut so that the carport has a fall for rain to run-off.
Transfer the markings from the front beam, where the battens will be attached, to the second beam, where the string line is. Measure the distance between the string line and the rafters, where the battens will be attached. These measurements are the height the battens need to be cut to.
Use the height measurements you have taken from each of the points along the string line and mark that distance on each of the battens you need to cut. Use the set square to mark a straight line along the length of each of the battens. Use the power saw to cut the battens along the line, so they're the correct height. Each batten should be lower than the other.