Take two big snowballs. Put them on the ground.
Then, get a bunch of snowballs and put them on top.
Next, you need some snowballs. Bigger ones than the other ones, but not as big as the other ones.
Now, take some snowballs and put them in a line, sticking them together.
Then, you need one big snow ball here and another one here.
Finally, make a pile of snowballs for ammo, and you have a snow trebuchet.