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Make a new psd (doh!), use the "spatter" airbrush at size 59. Airbrush the area you want the smoke at with white at about 50% opacity.

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Go to "filter ---> distort ---> wave" with the settings:

    »generators: 5
    »wavelength: min 10; max 120
    »amplitude: min 5; max: 35
    »type: sine

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After applying the filter, use "edit ---> fade wave", fade it to about 60%. Now move the smoke layer to another area.

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Repeat step 2 & 3 (wave/fade wave/move) many times (you don't have to use "fade wave" every time, skip it just a few times) until it gets a smoke shape. It's important that you move the layer every time or it will distort it in the exact same way.

When you are done you might want to erease some areas and apply an outer glow to it, depending how thick you want it.