Professional listening devices, also known as hidden microphones or "bugs," are small microphones capable of being hidden from plain sight while still being able to pick up conversations. These hidden listening devices are often employed by law enforcement, spies or private investigators for the purpose of catching people engaging in illegal or incriminating behavior. Installing professional listening devices requires clever use of objects to mask the microphone, as well as the equipment that is used to pick up the audio signal.
Scout the location where the hidden microphones will be placed. Plan out your installation carefully, especially if the space you are bugging is not one where you will have free access and you have only a short amount of time to hide the listening devices.

Choose your equipment carefully as smaller microphones will not be as good at picking up sounds but are more easily concealed. Find a place for the receiver. Decide if you will be able to plant a receiver/recorder combination somewhere in the space or if you will have to transmit the audio signal to an external location.

Position your receiver within range of the microphone but be sure it too is well camouflaged. Microphones only serve as a transmitter, and hidden microphones tend to have a fairly small transmission radius.

Install all of the listening devices at the same time, but do not leave the space until you have tested them all with a small receiver to ensure they are working properly. Cover your tracks by checking each place you installed the microphone for signs of being altered.

Tips & Warnings

Installing professional listening devices without proper authorization or probable cause may be a serious crime. Make sure that you have obtained legal authority to install these devices before you do so.
References
The Microphone Guide: Guide to Hidden Microphones
Granite Island Group: Warning Signs of Covert Eavesdropping or Bugging
VideoMaker: Hidden Microphone Placement