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.
Experiment with the exact placement of your garden arch. Use a peg and some string or twine to mark the spots for your four pressure-treated 4 by 4 posts. Tie a piece of string, the length of the distance between each post, to the peg. Hammer the peg into the first post placement. Pull the string taut, directly across from the first post and mark the spot. Pull the string diagonally to the third post placement, then straight ahead for the fourth post placement. Mark all four placements.
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.
Pour one bag of quick setting concrete into each post hole, then add water and mix. Check to ensure everything is level by placing the level on the top of the braces. Adjust with more water and concrete, if necessary. When concrete is set, remove the braces.
Measure and cut eight lengths of 2 by 4 foot cedar and eight lengths of 2 by 6 foot cedar to box in the four posts. Attach the 2 by 4 foot pieces to the faces of each post. Attach the 2 by 6 foot pieces to each side of the four posts. Use a pot, bowl or other round object to trace a curve on the ends of the four beams. Use a handsaw or jigsaw to cut the curves. Attach the crossbeams, two to each set of posts. These will be the sides of the arch.
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.
Secure the canopy to the top of the four posts by using the T-shaped metal brackets. There is no need to stain or paint cedar to protect it from the weather.
Benches can be added to the two sides of the arch. Lifting the beams with rafters and holding posts perfectly vertical and square will require at least one other person.