Choose a point from which you can see the entire floor and from which it extends longitudinally. This point may be in the main entrance, or it may be at one end of the room looking toward a feature, such as a fireplace.
Draw a straight line on the floor parallel to the back wall or the door threshold, using a straightedge and a pencil. Using a carpenter's square, draw a line perpendicular to that one, beginning at the point at which you're standing. Extend this line to the opposite wall with a chalk line.
Measure the distance from one end of the line to the wall against which you're going to start the installation, then make the same measurement from the other end of the line. Calculate the slant of the wall by subtracting the shorter measurement from the longer one.
Install the first row of flooring along the wall, and push all the boards a uniform distance of 1/4 inch away from it.
Measure a distance equal to the slant from the corner that is farther from the line, and make a mark on the floorboard in that corner. Snap a line on top of the boards that stretches between this mark and the opposite corner of the wall.
Put on safety glasses. Remove each board in the row, cut it along the line with a circular saw, and set it back in place 1/4 inch away from the wall.
Install the flooring until you reach the last row. Measure the gap for each board that goes in the last row with a tape measure, and cut the board with a circular saw.