It's fairly simple. Just make sure there's enough light at the location you're shooting in and use a high shutter speed to freeze the motion of the 'flying' gown. Or use slower shutter speeds for motion blur panning effects. Use flash if necessary to freeze motion, or with slow shutter speeds for partially frozen and motion blur combinations. Can look very nice if done correctly.
Got many ways wan ah. Hold/toss up and let go the simplest but goves the bride/photog the least amount of time to work with. If you want more long time flying to give the bride more time to move and create volume/swirl pattern etc, have to start with the end of the gown at a much higher elevation and bride must be comfortable turning around in her gown like a dancer, but no need to turn too much - about a quick 1/4 turn can. Have to trya few times to get right. . :sweat::sweatsm: