Take the 2 2x4x18" s and a protractor. Lie them on their thin edge and measure 2 1/2" down from the 'top.' Then, below that mark, measure another 1 1/2" and mark. These are the starting points for your cuts.
Turn the boards 45 (make sure you really turn one board 135 degrees, so the stand is symmetrical) using those marks as axis (Axises? Axii?) and mark the boards in a straight line for the cuts.
These will be your mounting notches. Again, make sure those marks are parallel on the each board, but when the boards are next to each other, the sets of marks are perpendicular.
Flip the boards on their fat sides again, but make sure those marks you just made are facing you. Measure 2 1/2" into the width of the boards, and mark a long line. This will be your 'stop' line.
Finally, with the 2x4x18's on their thin side, measure a 45 degree angle on the short-side corner opposite the marks you made for your notch. The resulting line should be one that runs perfectly perpendicular to your notch lines.
Now on the 2x4x6's, mark the boards on their thin side at 2" and 3 1/2." Then, flip it on the fat side and mark a stop line again on 2 1/2."
That was a lot of lines and marking. I'm sorry. Now we play with power tools again, I promise.