After your lines are in place (see part 4), you can start to cut the border tiles.
Cut the Tiles
Cut them face up with a sharp utility knife. Include the face and flange of the ceiling tile in your measurement. Because the walls are often uneven, each border tile should be cut and measured individually. Cutting these tiles 1/4-inch shy of the actual measurement will make them fit better. The gap between the wall and tile will be covered later by molding.
Cut the tiles so that the cut edge is against the wall and the factory edge points toward the interior of the room. Make sure the outside edges of the flanges line up exactly with your reference strings. Fasten each tile with four 1/2-inch or 9/16-inch staples, two in each flange (use six staples for 12″x24″ tiles).
Install the Tiles
Work across the ceiling, installing two or three border tiles at a time, and fill in between with full-size tiles. When you reach the last row of tiles, measure and cut each tile individually. If the stapling flange isn't large enough for stapling, face nail the tile in place near the wall, where the nail head will be hidden by the molding.