Lay a 9-foot board on the ground. Measure 3 feet from one end and mark the spot. Using a ruler, extend this mark to make a line on one side, and then to the remaining three sides. Also measure and mark a 3-foot height on the second identical length of lumber and extend to form lines on all sides.
Dig a hole over a sunny spot (one that receives six or more hours of sunlight every day) that is 3 feet deep and 8 inches wide. Level the base of the hole. Collect the soil in a tarp or wheelbarrow. Measure 10 feet away and dig a second hole with the same dimensions over the spot.
Pour concrete and water in a bucket, mixing well with a stick to get an ice-cream like consistency. Follow manufacturer's directions.
Stand the 9-foot lumber into the first hole, keeping the marked 3-foot section underground. Add concrete into the hole until it's several inches from the top. Leave overnight to dry. Repeat the procedure to stand the second length of lumber in the other hole.
Pour soil over the cured concrete into both holes. Do this until you reach the top of the holes so each is at the same level as the surrounding soil. Tamp the soil down to remove any trapped air bubbles.
Hold an end of one of the 12-foot poles and have a friend hold the other end. Position it horizontally 6 inches from the tops of the vertical poles. Align this horizontal board so a foot overhangs from each side, and drill wood screws to secure it in place.
Position a second identical pole horizontally over the previous one, but a foot below. Secure it to both vertical poles by drilling wood screws. Also secure the remaining two 12-foot lengths to the vertical poles. Space each a foot apart.
Secure both the 10-foot poles to the vertical posts in the form of an X shape. To do this, secure the top of one pole to the top of a vertical post with wood screws. Spread it diagonally so it meets the bottom of the second vertical post. Drill wood screws to join its lower edge in place to the second vertical post. Repeat this procedure to attach the top of the second pole to the other vertical post, and its lower edge to the bottom of the first vertical post.