Measure the width of the basement window opening with a measuring tape. Transfer this width onto a piece of plywood and mark it in several locations along its 8-foot length. Lay a 2-by-4 along these markings and draw a line along the 2-by-4's edge with a pencil.
Position the plywood on a workbench with the pencil line just over the edge. Cut along this line with a circular hand saw.
Position two 8-foot 2-by-4s under the cut section of plywood. Align each of their outside edges with the outside edges of the plywood. The plywood and 2-by-4s should form a "U" shape. Secure the 2-by-4s to the plywood with a 3 1/2 inch-wood screw every 6 inches, through the plywood and into the 2-by-4.
Push the trough through the basement window opening so that 1 to 2 feet of it extends out the window.
Stack three to four cinder blocks on the basement floor under the trough, about 1 foot from its end. Position the inside end of the trough on top of the cinder blocks.