Per my experience, it's all about the condition of the snow. Dry snow works better (but it also dissolves more quickly, so you have to hold back on the milk at first. Wet snow is more crunchy and not quite as pleasing.
Just fill up a one-person-sized bowl with dry snow, add some milk, sugar and vanilla and stir gently to combine, not adding too much milk so that the snow melts, and taste. Add more *whatever* if needed. I always just eye-balled it. I think it just takes some practice.
With acid rain and such these days, I'm not sure it's a great idea now, but it's created some great memories!