Use the posthole digger to dig four holes that are 1 foot deep. Place the holes in a square shape to form the basic structure of the arch; the holes should be four feet apart from one another. Each hole should be placed at a corner of the square.
Add 3 inches of gravel to the bottom of each hole to form a place for the poles to sit. This will also create a secure base once the concrete is added into the holes.
Place the 8-foot pieces of lumber into the spaces dug by the posthole digger. Ensure that each post remains vertical to avoid building a tilted arbor. Use spare lumber or other items to keep the posts vertical.
Add the quick-dry concrete to the holes, and mix in water according to the manufacturer's instructions. Allow the concrete to dry for at least 24 hours or according to the directions on the package.
Place the ladder between two of the posts to begin attaching the arbor top. Use 4-inch nails to attach one of the 4-foot-4-inch posts to the top of two of the 8-foot posts.
Repeat step 5 with the remaining 4-foot-4-inch posts to create a frame around the top of the 8-foot posts. This will create a place to attach the arbor slats.
Place one of the 2- by-4-inch boards a foot from the edge, and attach it with 4-inch nails at each end.
Repeat step 7 with the remaining 2-by-4 inch boards. Each board should be a foot away from the closest board to create an evenly spaced arbor top.