Cover an outdoor area --- or your garage floor --- with newspapers, plastic and an old blanket or a tarp. Flocking is a messy business.
Place your tree upright in its stand and in the center of the newspapers. Protect your eyes with goggles, and wear a mask if you are using commercial spray-on flocking. Slip on some gloves to protect your hands.
Brush or spray the flocking on the tree. Start at the top --- use a ladder if needed --- and work your way down and around the tree. Apply the flocking only to the tops and tips of the branches, just as if snow had landed there. Use large, sweeping motions if you are spraying it on, using a can of commercial flocking material. If you are painting the tree, lightly dab the branches with your paintbrush.
Let the flocking dry for a couple of hours. Apply a second coat if the look is not strong enough for your tastes.