Mark the locations for the four support posts on the ground; these locations should form a 10-foot-by-20-foot rectangle. Dig holes at these locations 24 inches deep and 12 inches wide. Compact the soil at the bottom of each hole using a hand tamper, add a 4-inch layer of gravel, and compact again.
Measure and cut 4-by-4 lumber for the support posts. These should be cut to size for your pergola's planned height. Prepare the ready mix concrete, set the posts in the holes, and add the concrete. Plumb the posts and temporarily brace them. Let the concrete cure overnight before continuing construction.
Measure and cut four 2-by-6 pieces of lumber to a length of 20 feet for the pergola beams. Attach decorative pergola ends to each end using heavy wood screws. Place these two beams on opposite sides, and 6 inches from the top of the support posts. Use the carpenter's level to make sure that they are both level. Secure them by screwing them into the support posts with heavy duty 3-inch lag screws. Repeat this process with the other two beams on the opposite posts.
Cut crossbeams for the pergola from the 2-by-6 lumber. These should be 11 feet long to account for the 10-foot dimension of the pergola, plus 6 inches overhang on each side. Again add pergola ends to the cross beams.
Make two sets of 1-1/2-inch notches in the cross beams using a table saw and a dado blade. The notches should be made in a location on the cross beams so that the cross beam will be able to slide over and onto the pergola beams. Lower the crossbeams into place over the beams and then secure them with wood screws.
Cut the top slats for the pergola by cutting 2-by-4s to a length of 11 feet, just as you did with the crossbeams. Use the table saw and dado blade to again cut notches in the slats, slide them over the beams, and attach them using wood screws. While doing this, place the slats at intervals of your preference to give the pergola the look you want. Apply stain, water seal or paint to the lumber, and your pergola is complete.