How to Make a Unicycle Out of a Bike
Remove the front fork. The front fork of a bicycle is the post that runs from the handlebars down to the front wheel. This is what you are going to use for your seat post. To remove the fork from the rest of the bicycle, take your grinder and grind off the frame where it is attached to the fork.
Remove the handlebar from the front fork. This may take some finessing, especially on an older bike. If you have trouble removing the handlebar, try applying bike lube to the joint were it is connected.
Remove the rear wheel and crank. To do this, remove the chain from the gears of the rear wheel and unscrew the bolts that hold it to the frame.
Attach the front fork to the wheel axle. There are a variety of ways to do this, and your method will depend on the specific fork and wheel you are using. If you are working with a fixed wheel, you can clamp pillow blocks on each side of the fork and then attach them to the axle at the spacers, using a bottom lock nut.
Insert the seat post into the front fork. If you want to be able to adjust the height of your seat post quickly, you may want to consider buying a quick release seat. These can be loosened and adjusted by hand unlike most seat posts, which require a wrench. Once your seat is in place, you're ready to unicycle.