Take a diamond hand pad or sandpaper and clean up any sharp edges.
The top surface will be a bit rough, and if you don't have a 5" orbital polisher, you might try sanding it with 60 grit sandpaper on a rubber pad or fixed to a block of wood. It will take forever but you can get it looking decent if you're patient. You can also grind and polish concrete using the same equipment you would use to grind and polish glass. At this step you'll get the best results if you have the right tool for the job (5" orbital polisher
with pads for concrete).
(Optional) Seal the concrete with something like the CHENG Acrylic Sealer.
Sealing concrete is a whole other thing, but there's some step by step information at that link. Sealing isn't necessary but it will protect the concrete and keep it looking good over the long term.
(Optional) If the cylinder is going to be a planter, drill a 1/2" hole in the bottom with a masonry bit.