We'll start off with a blank 400x300px canvas, and fill with the gradient of your choice:
Next, make a selection in the shape of a Christmas tree, something like this:
Next, choose a star shape from the Custom Shapes tool, and in a new layer, make all different sizes of it inside the tree selection. The Custom Shapes will be Paths, so when you're done, right-click the path in the Paths Window (Window > Paths) and choose Make Selection. Now you can fill in the shapes with the color of your choice:
Open up the Layer Styles for this layer and choose an Outer Glow.
Next, add a star Custom Shape to the top of the tree. Then add an Inner Glow layer style, and reduce the Fill opacity down to 0%.
Now we will create snowflake brushes. Create a new document about 50x50px, then draw a Custom Shape of a snowflake:
Make the background transparent and Select All (Ctrl+A or Select > All). Now goto File > Define Brush Preset. Give it a name and press Ok. This will now be available in your brushes. Repeat this step for all the snowflake custom shapes you want.
When you have all your brushes, go back to the Christmas composition and choose the brush tool. Select one of the snowflake brushes and then goto the Brush settings. Play with these values to achieve the look you desire. First, change Spacing to 200% or more, then check Shape Dynamics and set Size Jitter to 75% or so, Scattering - around 400%, both axes; and Other Dynamics - Opacity Jitter to 100%.
Add a new layer to the bottom of the composition, above the background layer. Now paint on the snowflakes - it may take a few tries, as positioning will be random:
Experiment and see what you can come up with. As an added touch to this one, I added green streamers hanging off the tree using the Pen tool and Stroke, and added the lightburst effect to the star using a Custom Shape:
More Tutorials: 
