Measure 6 inches in from one side of your piece of plywood. Draw a line from the edge to the 6-inch mark on the tape measure. Then, slide the tape measure 8 inches down the edge and make another 6-inch line.
Draw a line connecting the ends of the 6-inch lines, using a ruler to ensure it is straight. One inch in from both ends of each line, make a hashmark. Then, use the ruler and pencil to make straight lines from the hashmarks to the points where the 6-inch lines hit the edge.
Cut the wood along the diagonal lines just drawn. Cut along the connector line between the hashmarks; a trapezoid will result. The edge and the connector line are parallel, because they are always 6 inches apart. The other lines will not be parallel.
Sand your trapezoid as needed to get rid of splinters and rough edges.