Lavs come with two very similar looking pieces, a transmitter and a receiver.
Receiver is the end that will plug into the Audio Mixer
Here is the Receiver plugged into an audio mixer. It will also have the RF on the front of the receiver.
The Transmitter is connected to the microphone placed. Start with the mic about 8 inches below your talent's chin and adjust it from there until it sounds natural in your headphones.
Make sure both units have 2 AA batteries each.
ON light turns red for ON
Out of the box, the transmitter and receiver should have matching frequencies, meaning they can talk to each other wirelessly
To Change the frequency
Press up/down buttons to select "Frequency Preset"
Use the Up and Down arrow buttons to change the frequency
To Set frequency hit SET
then hit the on/off button to get out of the menus.