This seems to be a very easy questions. You steer to the left, you ride to the left. You steer to the right, you ride to the right. Wait, wait! How to steer a unicycle? This question was discussed in a thread in the German unicycling forum. It turned out that most people have not yet thought about it, albeit most unicyclists are able to ride curves, and they do it all time.
Possible answers may be available in a recorded lecture on mechanics at MIT, where phenomenons based on Newton's mechanics are being explained using a unicycle for visualization. How to initiate curves with a unicycle is being answered by countersteering like with a bicycle. A bicyclists doing a right turn uses the handle bar to turn the front wheel left, initiating a left curve, being tilted to the right by centrifugal force, straightening the handle bar (or maybe turning it slightly to the right), and is now in the middle of a right turn, being tilted appropriately, so that the sum of all involved forces FR exert along his body.
These forces are
The resulting riding path looks like this:
The fundamental challenge is to tilt inward to being able to ride the curve in a tilted position. A unicycle only has one single contact point with the floor. So doing turns like with a bicycle will not work with a unicycle, as starting the initial left turn is not possible. Actually unicyclists use a complete different technique for riding curves.
A unicycle becomes instable when tilting it to the front or to the back, as rotation around the body axle lowers the point of gravity. So gravity tilts the unicycle when it not balanced. From the following figure you can see how an upright unicycle (left) is being tilted around the wheel axle (middle), and then rotates around its vertical axle (right), lowering its point of gravity. The blue plane indicates the level of the wheel axle of the right unicycle, which is significantly lower than the wheel axles of both other unicycles.
By breaking or accelerating you can tilt a unicycle to the front or to the back. When riding straight it is a challenge to break balance in order to tilt the unicycle to one side or to another. This can be accomplished by a rotation movement perpendicular to riding direction. Based on conservation of angular momentum, and using static friction of tire and floor, the unicycle together with the rider tilts to the right when the rider makes a counter clockwise rotation movement, for instance with the arms. When watching beginners riding curves you can see them jerking their arms heavily. Advanced riders make pretty small movements that are almost invisible. You don't need to rotate your arms, but a small hip translation does the job.
Unicyclists can feel these steps that lead to riding a curve when they pay attention. Curves can be initiated by accelerating as well as by breaking.