First select an image of your choosing. In our case, it’s a basic water crystal with a Christmas tree inside.
Create a “snowflake” shape using the custom shape tool.
Press the Ctrl key and click the layer thumbnail to get the selection. From the edit menu, select define brush. Name it “snow_flake” just to make it easy to remember.
Select the brush tool. Press F5 to open brush settings. Select the brush and apply settings as shown below.
Create a new layer and apply the brush all over the image where suitable. This is our handy work below.
Create a selection of the dome using the lasso tool. Click add vector mask button from the bottom of the layer palette. Change the layer mode to “Overlay”.
Open the animation window. For the first frame without moving anything just unlink the content and mask the thumbnail in the snow layer using the layer palette.
Duplicate frame. Move the snow downwards.
Click the tween button from the animation window. Apply settings as shown.
You should have a nice set of frames for the animation done up by now. See below for an example.
Press Ctrl+Alt+Shift+S to save an optimized animated GIF of your work. Anyway, here is the final result.