This construct does not require a visual image, but you may use one if it is comfortable for you. To begin, try to reach out with your mind and encircle the energy of the room, or whatever local environment you are presently located in. Hold the energy in an energetic 'grip' so that you may exert some influence on it. The next step varies from individual to individual, but I will provide my own variant.
The construct itself requires a feeling of peace, of calm, and of relaxation. For me, I take a deep breath, and exhale strongly and slowly. Your energy will act upon that of the room, and in effect, take a 'deep breath' for the room. Following this, I use a hand motion to help with the intent, in which I hold both of my hands level, and slowly lower them to decrease the frequency of the energies' vibrations. This will slow down the reactivity of the energy in the room, and will cause less feelings of anxiety, of contest, or of anger.
This will take a deal of practice, but it is one of the more quickly learned constructs available. It can be used on groups large and small, and especially well for yourself. Just take a deep breath for all involved and things should smooth out nicely.