5. The snow golem should spawn snow under its feet (as of 1.5 this doesn't happen in desert, extreme hills, or extreme hill edge biomes). Observe that if you dig the snow under his feet, the snow respawns instantly. You can hold down dig to continuously generate snowballs.
6. Keep doing this, and you will have unlimited snowballs (at least, as long as your shovel holds out). Snowballs can be used as a projectile or to make snow blocks. (There is a slight bug where if you dig at just the right angle, you'll dig at an accelerated rate causing your shovel to be used up very quickly while creating massive amounts of snowballs.)
Video tutorials
NiceMarkMC's Automatic AFK Snow Farm Tutorial (for mass production of snow over long periods of time)视频(在 YouTube 上观看)