Be sure to follow the specific instructions with the unit you buy. Regardless of the manufacturer's instructions, if the fan blades are less than a screwdriver's length away from the ceiling, it may be best to install the blades before hanging the fan. The hanger pipe is usually placed into its hole on top of the motor. The wires are drawn up in the center. A set screw is tightened securely to make sure the pipe stays in place after it is threaded down.
Some fans have a separate motor hub into which the hanger pipe mounts.
In this case, you'll place the actual motor housing over the hub. Other fans have a two-piece decorative ceiling cover to hide the hole in the ceiling. Tighten the set screw well. To attach the fan blades, set the motor unit down where it will be stable. Often, the styrene foam packing for the motor housing makes an excellent stabilizer on your worktable.