Plant stakes in the ground at the corners of an 8-foot-by-10-foot rectangle. Tie string between these stakes. Use the carpenter's square to ensure that the corners of this rectangle are perfect 90-degree angles. Determine which half of the rectangle is higher and stand by the lower half. Take the string off the stake on the lower half and lift it. Place the level on the string and lift it until the string is level. Record the height at which the string is level. For this example, the slope comes down 2 feet, so you will have to compensate by making the lower half 2 feet taller.
Dig holes 8 inches wide and 12 inches deep with the posthole digger, centering on where you put the stakes. Place a plastic tube form around the edge of each hole, pushing it gently into the dirt.
Pour concrete mix into the wheelbarrow. Do this one bag at a time, as it will become heavy. Mix it by adding water, following the proportions on the packaging, and stirring with the shovel in a figure-8 pattern.
Pour concrete from the wheelbarrow into the holes you just dug. Allow the concrete time to dry, as directed on the packaging. After the concrete has dried, remove the tube forms. Drill holes in the concrete footings that line up with the predrilled holes in the post anchors. Use a bit that is slightly thinner than the masonry screws you will be using. Attach post anchors to the concrete footings by putting masonry screws through the predrilled holes in the post anchor, into the holes you drilled.
Cut two 6-by-6 piers to 2 feet long. Fix them in the post anchors on the lower side of the deck by placing the square end into the post anchor and putting nails through the predrilled holes in the post anchor, into the piers. Do this on either side.
Cut two 4-by-6 beams to 10 feet long. Lay one end into the post anchor on the taller side of the slope. Attach the other end to the top of the pier by putting a metal bracket between the pier and the beam and hammering nails through the predrilled holes, into the wood of the beam and the pier. On the other end, hammer nails through the predrilled holes in the post anchor into the beam.
Cut 2-by-6 boards to 8 feet long. Mount these joists between 10-foot beams by laying the joist on top of the beam, putting a metal bracket between the beam and the joist, and hammering nails through the predrilled holes in the metal brackets into the wood. Attach a joist every 16 inches along the beams, measuring from the center of one joist to the next.
Cut 2-by-5.5 boards to 10 feet long. Lay these across the joists perpendicularly. Nail them into the joists by hammering two nails through the board, into the joist, at every point the board crosses a joist. Start from one side and work your way over, keeping the boards aligned tightly.
Straighten the edges of the deck by cutting a straight line with the circular saw where the ends of the decking boards hang off. Sand the deck with the power sander, starting with a coarse-grit sandpaper and moving to a finer grit. Finish with a waterproofing sealant in a color of your choice.