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There are two basic ways to free mount.
In the first method you tuck the seat under you and then place your dominant foot on the pedal. The pedal should be pointing somewhere between you and the ground, about 45 degrees is good. Place pressure on the pedal causing the unicycle to roll backwards. This will bring the wheel under you. Pause momentarily, then ride forward.
The second method begins in the same way but you rely on the non-dominant foot. Applying just a little pressure with the dominant foot to keep the unicycle in place, launch yourself off the ground and forwards using your non-dominant foot and then ride forwards. This method has the advantage that you don't need to do a quarter revolution backwards first and you are imediately in a good position to ride off.
Make sure you are aware of which method is working for you and practice it.