Make sure every part of your picture is connected when drawing. If it isn't connected, it is more likely to fall apart before it floats.
Let the picture dry for a minute or so before adding water.
It doesn't matter if the water is warm or cold, but do not pour the water directly onto the picture. Pour it off to the side then wait for the picture to lift off the bottom of the dish. Once it is floating, then you can gently start moving the dish to watch your picture float.
Have fun!