See Below

Wait for a snow storm (this is the most difficult part) Put out a
bowl and let it fill up with fresh snow (you can also just scoop up
freshly fallen snow, but the standard warnings about tinted snow
apply :^) I think we use a two quart metal bowl. Add milk or
half-and-half (I prefer the latter) until you have a thick slush. I
add a little milk, stir, and repeat until you have the right
consistency. Add several tablespoons of sugar and several of vanilla
extract. These are both “to taste”. Stir. Adding the vanilla might
melt enough of the brew to make it too thin, so add some more snow as
needed.

Thats it. I know the above is not very precise, but thats half the
fun of it.

We have also added other things like semi-sweet chocolate chips or a
couple of tablespoons of unsweetened cocoa for variety.