Snowman seeds have to be found during winter. Occasionally, during winter of course, you may come across little balls of snow on the ground as you explore around the map. Get close to them and use the action button to forage and you may find a seed. Each seed will grow one Snowman.
Snowman are a.... Well a Snowman.
Harvest with: Scythe
There are many factors that affect growth time. But generally Snowmen should take about 7 to 10 minutes to grow IN SEASON. Consult your Farmer's Almanac if you have questions regarding seasons.
Snowmen grow fast, and need lots of water.