Add snow to a forest photo

I'm creating different seasonal/timed slides of the same photo of a forest scene. Managed to do spring, autumn/late afternoon & night-time myself, but cannot get anything close to a realistic snow-covered scene. I can provide the original photo plus the modified versions I've made if required, and a 'target' photo (i.e. approximately what I'm aiming for): Original is here: [login to view URL] And the kind of thing I'd like it to look like: [login to view URL]