Flocking a christmas tree is just adding fake snow. I personally just buy the cans and spray the tree myself which i purchase at walmart
This post might help
You can buy a spray, but be sure to do it outside and be careful where you direct the can.......
Spray the tree with water and use a sifter to add flocking to the tree. Repeat as necessary to get the coverage you want. It's a really messy process so protect your floor or work outside.