Preparation
Using measuring tape, get the exact length and width of the room where the ceiling will be installed. Draw the exact dimensions to scale on standard graph paper. Multiply the length and width together to calculate how many square feet you need to cover.
Drop ceiling tiles are primarily available in two sizes: 2-feet by 2-feet, or 2-feet by 4-feet. After you decide on a preferred tile, you can purchase the appropriate amount of tile, hanging brackets and grid-work based on your previous calculations of square footage. Use the graph paper to draw a test layout to confirm that you are purchasing the correct quantity of ceiling tiles.
Installation
A drop ceiling should be hung about 4 inches from the ceiling joists, this will provide enough space to angle the tiles into place. Use a 360-degree laser level to mark a line that circles the entire room 4 inches below the existing ceiling joists. The laser level will ensure that your ceiling is perfectly level, even if the structural ceiling is not.
Install the perimeter hanging brackets around the walls of the entire room, closely following the level lines you marked.
Hang the main runners from the ceiling joists at a distance of 4 feet apart using 14-gauge wire connected to eye bolts screwed into the joists. They should span the entire room running perpendicular to the joists, at the exact height of the perimeter hanging brackets on the walls.
Hang cross-runners perpendicular to the main runners every 2 feet, securing them with ties at every intersection with the main runners.
The tiles are now ready to be lifted and dropped into place. Use a utility knife to trim the edges of any ceiling tiles that are too large.