Your Therapist will spend some time doing some relaxation exercises with you, which could include 'safe or pleasant place' exercises, guided visualisation, deep muscle relaxation, breathing retraining, imagining resources etc.
Once you and your therapist feel that you are sufficiently prepared, you can then target a distressing memory with the eye movements or other forms of left-right alternating stimulation, such as sound or taps.

Your therapist will ask you to select an image that represents the distressing event.
You will then be asked to think about negative thoughts in relation to the memory and to think of an alternative positive thought that you would like to believe about yourself.
You will also be asked about your feelings, the amount of distress you feel and where you feel it in your body.

Your therapist will then begin the eye movements while you bring to mind the picture, feelings and bodily sensations and the negative thoughts or beliefs.
The therapist will move their fingers rapidly from side to side while you allow you eyes to follow the movement or may gently tap on either side of your knees or on your hands. While following the movements you allow your thoughts to follow where they need to and just let whatever comes up, come up without censoring.

After each set of eye movements your therapist will ask you what came to mind or what you noticed during the eye movements. During the eye movements you may experience the distressing event quite intensely to start with, but this distress generally reduces as the memory is processed with EMDR. You are free to stop at any time, just raise your hand to indicate you want to stop.

Your therapist will continue with the eye movements until your distress is reduced as much as possible. Your therapist will then ask you to think about your positive thought and also check whether there is any part of your body where you still feel distress. Before the end of the session, your therapist will give you time to feel calm again, using the safe-pleasant place exercise or relaxation techniques.

The important thing to remember is that it is your own brain that will be doing the healing and that you are the one in control.

You might like to watch this Animation explaining what happens in EMDR therapy