Bathroom extractor fans pull moist air out of the bathroom to the outdoors to keep it from remaining trapped in the room, where it can foster mould growth and cause other problems. There are several ways to install a new one, some of which require a professional electrician (such as installing one in an exterior wall where there isn't currently a power source). If the bathroom has an existing ceiling light fixture and an unfinished attic above it, you can fit the new fan into place yourself.
Cut the electricity to the old ceiling fixture at the fuse box. Remove the mounting screws that hold it to the ceiling. Pull down the fixture to reveal the wiring. Disconnect the wiring, leaving the ceiling electrical box exposed.
Locate the electrical box on the floor of the attic over the bathroom. The wire cable from the house will be running to it. Loosen the clamps on the box with your screwdriver. Pull the wires free. Set the wire cable aside, still connected to the house circuit.
Remove the screws that hold the electrical box to the floor. Remove the box. Set the extractor fan over the hole that looks down into the bathroom, covering it completely with the vent of the fan.
Mark around the perimeter of the vent, onto the floor. Remove the fan. Cut around the line with a jigsaw. Set the fan back in place so the vent portion drops through the hole.
Set the mounting brackets of the extractor fan kit against the joists on either side of the fan. Mount the brackets there with the screws provided with your kit.
Connect the vent hose of the kit to the fan according to the instructions. (This will usually be accomplished with a metal or plastic band that wraps around the hose onto the exhaust hole of the fan.) Hold the other end of the hose to an exterior wall. Mark around the perimeter of it. Cut it out with your jigsaw. Mount the hose to the wall with the provided screws.
Hook the wires from the cable that's still attached to the house circuit to the wires that are coming out the top of the fan. Twist wire caps around the wires, joining the black wire from the cable to the black wire from the fan. Connect the two white wires in the same fashion.
Hook the bare grounding wire from the house circuit cable to the green grounding screw in the fan.
Install the vent cover over the hole in the ceiling. Connect it to the vent opening with the spring hooks that came with the kit.
Turn the power to the circuit back on. Test the fan at the wall switch.