When using wood to build outside accents, naturally durable wood like red cedar or wood with pressure-impregnated preservatives should be used to increase the life of the project. A generic plan may work well for the area you have in mind for your garden arch, or you may have to adjust the height, length or width. This can be done and still allow you to follow a given plan. Measuring the area where you intend to build your arch will give you the measurements you need to take to the lumber yard.
Use a framing square to ensure 90 degree angles. Bore four holes, 42 inches deep. Pour 6 inches of crushed rock into each hole. Place the first post in the hole and use a level as you attach a brace to this post and the adjacent post. Attach a brace to all four sides. Check for 90 degree angles on each corner.
Trace the arch you prefer onto two beams that are 12 inches wide. You will trace the top and the bottom of the arches. Cut the arches using a skill saw or jigsaw. Use the power sander if you prefer a smooth cut. Starting in the middle, attach the rafters to the arched beams using a spacer to maintain equal distance between slats. Attach the beams and canopy to the posts by driving angled screws into each corner. You should have the tops of all four posts cut by handsaw to be flush with outside casing of posts.