I have an image sequence of snowflakes - I want them to be only seen inside the letters "SNOW" (separate layer) and no where else. I do have other layers as well, so I didn't know the best method for achieving this result.

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Sorry, I forgot to add the text was created in Photoshop, I don't know if that makes a difference or not.

It shouldn't. Just make white text on a black background or black text on a white background.