Harvest when small in size and deep solid red color.
Precook until tender. Then cool. Remove skins and then slice or dice the beets into freezing containers. Small beets (about 1 1/2 in. in diameter) may be frozen whole. Seel containers & place in freezer without delay.

This is from the feezer instructions which came with my freezer. United Freezer Co. Hudson, WI.