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".