Begin by measuring across the front lentil and finding the center. Make a mark 3/4" to the side to give a guide for the edge of the 2 x 8. From here, mark a line every 16" across the front lentil. The rafters are going to span the distance of the lentils, and they're the pieces that will create the shade. And if it's shade you want from the rafters, they should run north and south. For this particular project, the rafters are spaced approximately 16" apart, which will supply good shade but still allow people underneath to see the sky.
Mark and cut the first rafter (Image 1). Set the rafter in place, carefully allowing for the overhang. Then pre-drill the rafter (Image 2). Toenail 3" galvanized wood screws into the lentil. When you're toenailing a screw, it's important to pre-drill the holes before you begin setting the screws.
To find the location of the rear lentil for the rafters, use a framing square. Once you located where the mark falls on the 2 x 10, measure the distance and be sure to add 10" for the overhang. Cut one end of each 2 x 8 rafter to get the necessary length desired. Make the final decorative-edge cut (using a template as before) with a saw, touch the end up with paint and set in place.
On to the next rafter, repeating the steps exactly as before. Continue until all the rafters have been secured. Touch up the nail holes by painting over them instead of using wood putty. It's easier and just as effective. Also, if you ever have a problem with a rafter, it can be removed easily.