With the two leg assemblies completed, we'll turn our attention to the beam, the main support on the project. This beam will need to be notched twice for each leg assembly, as the notches and cross braces will connect providing considerable lateral support for the stand. These notches will be cut as shown on page 3 of the free woodworking plans you downloaded in step 1.
To begin, measure in 30-inches from each end of the 12-foot long pressure-treated 2x6. Using your Combination Square, mark a 75-degree angle leaning toward the center (105-degrees from the opposite side of the angle, as shown in the plans). Make a cross-mark 2-1/2" up the line. At this spot, mark a 1-1/2" long line perpendicular to the angle. From the end of this line, make a line at a 75-degree angle (parallel to the first line) back down to the bottom of the beam. This will be the cutout for the first notch as shown on page 3 of the plans. Repeat on the opposite end of the beam.
Next, position one of the leg assemblies on the floor in the approximate angle that the unit will be used (with the bottoms of the legs flat on the floor). Measure the distance between the high points of the two cross braces on the top of the leg assembly (the distance should be around five inches).
Make a mark at this distance from the first mark you made in this step (30-inches from the end) and mark out a second notch on this end of the beam exactly as you made the first on this end. Repeat the procedure on the opposite side (being certain to measure the distance between the cross braces). The four notches should look just like they do on page 3 of the plans.
Now, use a cross-cut hand saw and cut the parallel lines of the four notches (eight cuts in all). Use a sharp chisel and ease out the ends of the notches. Take your time to make sure you cut the notches cleanly.