In this tutorial I'm going to look at using "Soft Flake Snow" *, "Water Effects" *, and PVA (white glue) to create realistic snow effects. All of these materials can be found in the Secret Weapon Instant Scenery Kit: Winter.
Although you will find several tutorials out there demonstrating how to use sodium bicarbonate (baking soda) to create snow the effect is not as soft, flaky or realistic as you can get from modeling snow. I have used six different modeling snows and the "Soft Flake Snow" is my personal favorite - which is why it is included in the kit.
I'm going to start with two finished Runic Mountain cavalry bases:

There are two basic ways to add snow to your base, miniature or model:
First Method:
Dabbing the adhesive (PVA or Water Effects) onto the base and then sprinkling "Soft Flake Snow" over the area:

On the left I have used PVA - on the right "Water Effects." Once the adhesive is in place I poured a heavy amount of "Soft Flake Snow" over the area. Doing this over a piece of paper that has been folded in half will allow you to easily recover the excess.

Second Method:
Mix the adhesive (PVA or Water Effects) and "Soft Flake Snow" prior to applying it to the base:

The mix ratio is really a matter of preference. If you want it to look a bit more wet, then use more PVA or "Water Effects". Adding a small amount of white paint to the mix will help reduce the wet, slushy look of the snow.
The end results:
First up is the "dab & sprinkle" method with PVA:

This gives us nice, slushy snow that lets a lot of the underlying color show through.
Next we have the "Water Effects" used with the "dab & sprinkle" method:

This has a colder look with less slush effect while also allowing a lot of the underlying colour to show through.
The PVA mixed with the "Soft Flake Snow" and applied with a brush gives us a much different look:

In this case the underlying color is only visible at the edges and the finished effect is more "dry" even though I didn't add any white paint to the mix.
Last, but not least, we have "Water Effects" mixed with "Soft Flake Snow":

This combination gives us a very wet, slushy snow and still shows some of the underlying colour throughout.
These techniques can be used together on your model, base or scene to create more than one effect. You might want to add slushy snow in tank treads and crisp, dry snow around it for instance. I recommend experimenting with each method and generally apply at least two to any model I'm doing.
If you have a question about basing, weathering or other detailing please email misterjustin at secret weapon miniatures dot com.
* "Water Effect" and "Soft Flake Snow" are copyright Woodland Scenics, Inc. Used with permission.