Start by building the sides. Cut a 1x3 to six feet, giving the cut edge a 22.5* angle. Then cut a second 1x3 to seven feet, again giving the cut edge a 22.5* angle.
Next, cut a 2x3 to two feet, and fasten it to the two longer sticks, six inches up from the edge. Notice that the two foot 2x3 overhangs the shorter stick by 1.75" - the same as the width of a 2x3.
Finally, checking carefully for square, place a 2x3 on top of the longer sticks, connecting them flushly. Scribe the angle of the cut on 2x3, cut it, and then fasten it to the top.