First, it looks like you are pedaling a bicycle with your feet. Make both hands into fists, then make vertical circles in the air in front of your body. Finally, hold up the number '1' on your dominant hand by raising your index finger while the rest of the fingers are curled in, with the palm side facing you.
It would take a great amount of fine motor dexterity and balance to ride a unicycle, so it would be best for your baby or toddler to simply admire unicycle riders from a distance first, without actually trying it for themselves -- unless you come from a line of circus performers, and passing on this skill would be a cinch!
If your little one shows some interest in biking, consider purchasing a balancing bicycle or a tricycle, which can be used as early as age two. A unicycle may suit them much later on still, once they have honed their balancing skills.
Get all four volumes of Baby Signing Time...