Now created a orthographic camera on top of your scene and make a render to use as a guide in photoshop

In photoshop you should not begin painting with a white canvas, as you will be unable to have negative displacement, so instead pick a color that will give you enough leeway in both positive displacement and negative displacement, it would be wise to study your scene to figure out what will be needed most

For this one i used a dark gray since i wont be digging much so R:77 G:77 B:77

Render for Photoshop

Then you just start digging into the snow removing where needed, try to think about how it should be, where are the hotspots that would leave less snow, what kind of shoes a person uses, and how he walks.

Obviously try to reduce the amount of snow where there is cover, or where there was cover, you can make a story with it eg: a car that was parked at one point then drives off leaves a distinctive mark in the snow.

Final Snow Displace

some places you need to add, pretty much anywhere buildup is possible mostly with static objects, a wall with the wind againts it will harvest more snow at it"s base

when you are done, add a noise map overlayed with the final snow displace, as snow is never perfectly flat when you look close, just to create diferent elevations.

Back into Max we start with two planes, the top one will be the standard ground positioned at Z:0.0 then make another plane just under it, tweaking will be necessary

I found Z:-0.4 was a good overall for me

I used a Vray shader for the snow, with some blurry reflections, and a low fresnel, i also added a bump speckle for some added detail

Plug your displacement map, i added some blur to it, some tweaking is always necessary, Sometimes the details done in photoshop are too coarse for real snow, or you do not have enough displacement triangulation to reproduce them as it should be.

Setup the Vray Displace map, click on the modifier list and selecet VrayDisplacementMod

Select 2D Mapping, and start tweaking the amount of displace, the resolution and precision work together, be carefull extreme values can significantly increase render time and may even prevent you from rendering due to memory swapping

Now render time, you will need to have some GI into your scene, snow is very reflective it needs light and an environement to shine, i tried recreating a light moon light over the snow. you can do that by either adding some direct lights pointing down, or by adding a skylight with a dark blue color

leave the vray displacement tab as is.