Most burglar alarm systems come with a series of door alarms, each consisting of two parts. The sensor itself is either mounted on the frame of the door or embedded directly into the wood. There's also a magnet--mounted or embedded on the door itself--which holds the sensor's trigger in place. When the door is open, the magnet pulls away from the sensor, activating the alarm. In most cases, you can mount a door sensor yourself, though wired systems may require a professional to connect it to the remainder of the system.
Wipe the door and doorway down with an all-purpose cleaner or rubbing alcohol to clear away any dirt. You want the work surface to be as clean as possible.
Mark locations for the sensor in the frame of the door. It should be high up and on the opposite side of the frame from the hinges. If you're mounting the sensor on the door frame, use a stud finder to locate the sturdiest spot. If you're embedding it in the door frame, it needs to be on a spot in contact with the door when it's closed and have enough free space behind it to fit the recessed sensor.
Place the alarm sensor in position and use a carpenter's level to make sure it is parallel to the floor. Mark the outline with a pencil so you know where it goes.
Drill screw holes into the spots on the door frame matching the appropriate locations on the sensor. In the case of a recessed sensor, you may need to drill a particularly large hole to fit it. Make sure the drill bit corresponds to specifications in the sensor's instruction manual.
Fit the sensor into place and screw it securely to the door frame. If you're using a recessed sensor, make sure it's flush with the frame so it doesn't interfere with opening and closing the door.
Find the spot on the door corresponding to the mounted sensor when the door is closed. This is where the magnet goes. Hold the magnet in position and use a carpenter's level to make it parallel with the floor. Mark that location with a pencil.
Drill holes for the magnet's screws in the door, or a hole for the magnet itself if you're using a recessed sensor.
Mount the magnet and screw it into position (or secure it to the recessed hole if you're using that kind of system). Check to make sure it's flush with the door and that it stays in contact with the alarm sensor when the door is closed.
Turn the components on and test them to ensure that they work.
While most door sensors use batteries, some are wired to the electrical grid in your house. Always turn off the power in the part of the house you're using before attempting to install those types of sensors.