Measure the distance between the edge of the last panel you installed and the top edge of the wall. Add 1/4-inch to this measurement.
Mark the width of an uninstalled panel to fit the above measurement, using a fine-point marker and straightedge. Cut the panel carefully along the line, using tin snips.
Tuck 1/4-inch of the panel’s edge underneath the last panel you installed so that the cut edge of the new panel is against the wall.
Nail the panels along the overlapping edges every 6 inches at a slight angle, using a brad nailer.
Place the wood block at a 45-degree angle against the ceiling panel's seam. Using the hammer, tap the wood block all along the seam to close any gaps.