Step 3: The dialog box for the Snowflakes plugin will now appear on your screen. Here, you can change several different settings in order to get the exact appearance you want for the snow in your digital photo.

click to enlarge
The preview window in the plugin lets you see the result of the proposed settings before you apply them to the actual photograph. This is nice because it lets you experiment and see several different possibilities before deciding on any particular one.
However, it is important to note that sometimes the results will look a little different on your photo than in the preview area because of the size of the photo. Although the settings might look like a very light snow effect in the preview window, they could appear more pronounced after being applied to the photo. So, you may have to practice and use the "undo" feature a few times to find the exact look you want.